12 Dec is in December.
On 12 Dec 884 Carloman II King West Francia (age 18) died. His half first cousin once removed Charles "Fat" King East Francia Holy Roman Emperor III King West Francia King Aquitaine (age 45) succeeded III King West Francia, King Aquitaine.
Before 12 Dec 1204 Waleran Beaumont 4th Earl Warwick (age 51) and Alice Harcourt Countess Warwick (age 53) were married. She by marriage Countess Warwick. He the son of Roger Beaumont 2nd Earl Warwick and Gundred Warenne Countess Warwick.
On 12 Dec 1204 Waleran Beaumont 4th Earl Warwick (age 51) died. His son Henry Beaumont 5th Earl Warwick (age 12) succeeded 5th Earl Warwick 1C 1088. Philipa Basset Countess Warwick by marriage Countess Warwick.
On 12 Dec 1212 Archbishop Geoffrey Plantagenet (age 60) died at Normandy [Map]. He was buried at Notre Dame du Parc, Rouen.
Before 12 Dec 1247 Richard de Dover Plantagenet (age 27) and Matilda Angus 6th Countess Angus (age 25) were married. She the daughter of Maol Choluim Angus 5th Earl Angus. They were fourth cousins. He a grandson of King John "Lackland" of England.
On 12 Dec 1290 Eleanor of Castile Queen Consort England (deceased) body rested at St Albans [Map].
On 12 Dec 1298 Albert Habsburg II Duke Austria was born to Albert Habsburg I Duke Austria (age 43) and Elizabeth Carinthia Duchess Austria.
Before 12 Dec 1306 Roger Bigod 5th Earl Norfolk (age 61) died. Earl Norfolk 2C 1141 extinct. In 1302 Roger Bigod 5th Earl Norfolk (age 57) had surrendered his Earldom to King Edward I (age 67) and was recreated Earl Norfolk 2C 1141 with the remainder "to the heirs of his body" effectively disinheriting his brother John Bigod. Its not clear why he did so.
Before 12 Dec 1325 Stephen Segrave 3rd Baron Segrave (age 40) died. His son John Segrave 4th Baron Segrave (age 10) succeeded 4th Baron Segrave 1C 1283.
On 12 Dec 1414 William Bonville 1st Baron Bonville (age 22) and Margaret Grey (age 17) were married. She a great x 4 granddaughter of King Edward "Longshanks" I of England. 
On 12 Dec 1417 John Talbot 2nd Earl of Shrewsbury was born to John "Old Talbot" Talbot 1st Earl of Shrewsbury (age 34) and Maud Neville 6th Baroness Furnivall (age 24) at Sheffield Castle [Map].
On 12 Dec 1448 John Talbot 3rd Earl of Shrewsbury was born to John Talbot 2nd Earl of Shrewsbury (age 31) and Elizabeth Butler Countess Shrewsbury and Waterford (age 24).
On 12 Dec 1534 Gerald Fitzgerald 9th Earl of Kildare (age 47) died. His son Thomas "Silken" Fitzgerald 10th Earl of Kildare (age 21) succeeded 10th Earl Kildare.
On 12 Dec 1549 Marguerite Valois Orléans Queen Consort Navarre (age 57) died.
On 12 Dec 1562 William Grey 13th Baron Grey of Wilton (age 53) died. His son Arthur Grey 14th Baron Grey of Wilton (age 26) succeeded 14th Baron Grey of Wilton 1C 1295.
On 12 Dec 1574 Thomas Fane (age 64) and Mary Neville 3rd Baroness Despencer (age 20) were married at Birling [Map]. The difference in their ages was 44 years. She a great x 5 granddaughter of King Edward III of England. 
On 12 Dec 1594 Edward Russell 3rd Earl Bedford (age 21) and Lucy Harrington Countess Bedford (age 14) were married at St Dunstan's Church Stepney. She by marriage Countess Bedford. She brought an enormous dowry of £10000 and the estate of Minster Lovell [Map].
After 12 Dec 1610. Memorial to Christopher Freeman died aged fifty-one. Brass plate with semi-kneeling figures of him, his wife, fgour sons and four daughters.
On or before 12 Dec 1639 Charles Boyle 3rd Baron Clifford was born to Richard Boyle 2nd Earl Cork 1st Earl Burlington (age 27) and Elizabeth Clifford Countess Burlington (age 26). On 12 Dec 1639 Charles Boyle 3rd Baron Clifford was baptised.
Pepy's Diary. 12 Dec 1666. Up, and to the office, where some accounts of Mr. Gawden's were examined, but I home most of the morning to even some accounts with Sir H. Cholmly (age 34), Mr. Moone, and others one after another. Sir H. Cholmly (age 34) did with grief tell me how the Parliament hath been told plainly that the King (age 36) hath been heard to say, that he would dissolve them rather than pass this Bill with the Proviso; but tells me, that the Proviso is removed, and now carried that it shall be done by a Bill by itself.
On 12 Dec 1671 William Seymour 3rd Duke Somerset (age 17) died. His uncle John Seymour 4th Duke Somerset (age 26) succeeded 4th Duke Somerset 4C 1547, 3rd Marquess Hertford 1C 1641, 4th Earl Hertford 4C 1559. Sarah Alston Duchess Somerset (age 29) by marriage Duchess Somerset.
Before 12 Dec 1676 Jacob Huysmans (age 43). Portrait of William Morice (age 74).
Before 12 Dec 1676 Jacob Huysmans (age 43). Portrait of John Middleton 1st Earl Middleton.
On 12 Dec 1679 John Crew 1st Baron Crew (age 81) died. His son Thomas Crew 2nd Baron Crew (age 55) succeeded 2nd Baron Crew of Stene in Northamptonshire.
On 12 Dec 1718 Marmaduke Langdale 3rd Baron Langdale (age 62) died. His son Marmaduke Langdale 4th Baron Langdale (age 33) succeeded 4th Baron Langdale of Holme in Yorkshire.
On 12 Dec 1738 Richard Molyneux 5th Viscount Molyneux (age 59) died without surviving male issue. His brother Caryll Molyneux 6th Viscount Molyneux (age 55) succeeded 6th Viscount Molyneux, 7th Baronet Molyneux of Sefton.
On 12 Dec 1751 Henry St John 1st Viscount Bolingbroke (age 73) died. His nephew Frederick St John 3rd Viscount St John 2nd Viscount Bolingbroke (age 18) succeeded 2nd Viscount Bolingbroke.
On 12 Dec 1752 Thomas Warren-Bulkeley 7th Viscount Bulkeley was born to James Bulkeley 6th Viscount Bulkeley and Emma Bridget Rowlands Viscountess Bulkeley.
On 05 Dec 1754 Henry Nassau 1st Earl Grantham (age 81) died without surviving male issue. Earl Grantham extinct. On 12 Dec 1754 Henry Nassau 1st Earl Grantham (deceased) was buried at St James' Church, Piccadilly.
On 12 Dec 1758 George Ludlow 3rd Earl Ludlow was born to Peter Ludlow 1st Earl Ludlow (age 28) and Frances Lumley-Saunderson Countess Ludlow.
On 12 Dec 1787 Francis Needham 2nd Earl Kilmorley was born to Francis Needham 1st Earl Kilmorley (age 39).
On 12 Dec 1805 Henry Devereux 14th Viscount Hereford (age 28) and Frances Elizabeth Cornewall Viscountess Hereford (age 22) were married. They had five sons and one daughter. She by marriage Viscountess Hereford.
On 12 Dec 1807 William Craven 1st Earl Craven (age 37) and Louisa Brunton Countess Craven (age 22) were married at his house in Berkeley Square, Mayfair. He a great x 3 grandson of King Charles II of England Scotland and Ireland. 
Archaeologia Volume 20 Section VIII. An Account of the First Battle of St. Albans from a contemporary Manuscript. Communicated by Joun Baytey, Esq. F.S. A. of His Majesty's Record Office in the Tower; in a Letter to Henry Extis, Esq. F. R. S. Secretary.
Read 19th December 1822.
His Majesty's Record Office, December 12, 1822.
DEAR SIR,
I inclose a curious Account of the first Battle of St. Albans, which may prove interesting to the Society of Antiquaries: it is copied from a manuscript, in a co-eval hand, found in the Tower, among a large quantity of private letters and accompts of Sir William Stonor, Knt [William Stonor?] who, from his correspondence, appears at that time to have been much about the Court, and was also a Steward of the Abbot of St. Albans. The original is written in a book consisting of a few leaves of a small quarto size, and, on comparing the writing with some of the other papers, it seems to be in the hand of Sir William himself.
I remain, dear Sir, your's very faithfully,
JOHN BAYLEY.
To Henry Ex is, Esq &c. &c. &c.
On 12 Dec 1825 Elizabeth Bentinck Marchioness Bath (age 90) died. Memorial in Saints Peter and Paul Church, Longbridge Deverill [Map].
Elizabeth Bentinck Marchioness Bath: On 27 Jun 1735 she was born to William Bentinck 2nd Duke Portland (age 26) and Margaret Cavendish Harley 2nd Duchess Portland (age 20) at Welbeck Abbey. In 1759 Thomas Thynne 1st Marquess of Bath (age 24) and Elizabeth Bentinck Marchioness Bath (age 23) were married. She by marriage Viscountess Weymouth. She the daughter of William Bentinck 2nd Duke Portland (age 49) and Margaret Cavendish Harley 2nd Duchess Portland (age 43). They were third cousins.
On 12 Dec 1837 Edward Bootle Wibraham 1st Earl Lathom was born to Richard Bootle-Wilbraham (age 36) and Jessy Brooke in Blythe Hall Lathom.
On 12 Dec 1839 Henry Roper-Curzon 15th Baron Teynham (age 50) and Sarah Rudd Baronss Teynham were married.
On 12 Dec 1860 Henry FitzWalter Plumptre 20th Baron FitzWalter was born to Major John Bridges Plumptre (age 28) and Elizabeth Wright (age 25).
On 12 Dec 1870 Beaumont Hotham 3rd Baron Hotham (age 76) died unmarried without issue. His nephew Charles Hotham 4th Baron Hotham (age 34) succeeded 4th Baron Hotham of South Dalton in Yorkshire, 14th Baronet Hotham of Scorborough in Yorkshire.
On 12 Dec 1914 Geoffrey Hope-Morley 2nd Baron Hollenden (age 29) and Mary Sidney Katharine Almina Gardner (age 18) were married. They were divorced in 1928.
On 12 Dec 1915 William Neville 1st Marquess Abergavenny (age 89) died. His son Reginald Neville 2nd Marquess Abergavenny (age 62) succeeded 2nd Marquess Abergavenny, 6th Earl Abergavenny, 2nd Earl Lewes, 22nd Baron Bergavenny 1C 1392, 20th Baron Bergavenny 2C 1450.
On 12 Dec 1916 Reginald Mowbray Chichester Huddleston (age 22) and Edith Maud Rawdon-Hastings 12th Countess Loudon (age 33) were married.
On 12 Dec 1923 Wentworth Canning Blackett Beaumont 1st Viscount Allendale (age 63) died. His son Wentworth Henry Canning Beaumont 2nd Viscount Allendale (age 33) succeeded 2nd Viscount Allendale of Allendale and Hexham in Northumberland, 3rd Baron Allendale of Allendale and Hexham in Northumberland. Violet Lucy Emily Seely Viscountess Allendale (age 26) by marriage Viscountess Allendale of Allendale and Hexham in Northumberland.
On 12 Dec 1940 Philip Kerr 11th Marquess Lothian (age 58) died at Washington DC.
On 12 Dec 1974 Elizabeth Frances Abney-Hastings Viscountess St Davids (age 90) died. Her son Jestyn Reginald Austin Plantagenet Philipps 2nd Viscount St Davids (age 57) succeeded 15th Baron Strange Knockin 2C 1299, 23rd Baron Hungerford 1C 1426 and 21st Baron Moleyns 1C 1445.
On 12 Dec 1991 James Stourton 25th Baron Stourton, 27th Baron Segrave, 26th Baron Mowbray was born to Edward Stourton 24th Baron Stourton, 26th Baron Segrave, 25th Baron Mowbray (age 38).
On 12 Dec 2006 Charles Stourton 23rd Baron Stourton 25th Baron Segrave 24th Baron Mowbray (age 83) died. His son Edward Stourton 24th Baron Stourton, 26th Baron Segrave, 25th Baron Mowbray (age 53) succeeded 24th Baron Stourton, 26th Baron Segrave 1C 1283 and 25th Baron Mowbray 1C 1129.
On 12 Dec 2008 David Charteris 12th Earl Wemyss (age 96) died.