Florence of Worcester Continuation. 20 Dec 1135. ... and Stephen (age 41), his sister's son, being elected to the kingdom of England, was consecrated king, by William (age 65), archbishop of Canterbury, on the thirteenth of the calends of January [20th December], at London, where he held his court, at Christmas, surrounded by the nobles of England, with great courtesy and royal pomp.

Note 1. Wikipedia states 22 Dec 1135 although doesn't provide a source?