800-900 AD. I presume you refer to the "Life of King Alfred," by Bishop Asser. This work is controversial, some claiming that it was written much later than the 9th century.
In any case, there is no mention of mummery here, at least in my quick scan of a modern English text.
In a thread last summer (49704), it was pointed out that Henry VIII disapproved, and that mumming seemed to have been known in England in the late Middle Ages, but no earlier mention was found.