I think at this point we need to clarify one or two things, if only to avoid confusing others. "Guest" and I have been talking about Mummers' Plays, of which there is no known record earlier than the 18th century. Dicho is talking about Mummers as a whole, which is not the same thing; the term is an old one and covers, so to speak, a multitude of sins. Certainly there are mediaeval references to Mummers; while they dressed up and performed in various ways, none of the many records indicate that they performed plays of any sort. The term is still also occasionally used in Britain (and more commonly in the USA, I think) to describe "luck-visitors" and the like who wear disguise but do not perform plays.Adolphus William Ward was born in 1837 and died in 1924; he seems to have subscribed to the "pagan origins" theory which was popular in his day. Some of the connections he makes in his article as quoted above would be rather suspect by modern standards, and don't relate to the Mummers' Play at all; he seems to have assumed that "mummers" always referred to the same thing (it didn't; it's a generic term meaning more or less "any person who performs in disguise"), and that the simple use of the term implied some sort of link or continuity between all instances in which it occurred. The same seems to be the case with the Book of Days reference that Masato gave earlier on.
In fact, not all performers of the seasonal folk plays are called, or call themselves "mummers", anyway. There are Soulers, Pace Eggers, Plough Jags (or Stots) and so on. In the North of England a form of the play is often associated with the sword dance, but this is not the case elsewhere. Quotations from old works of general reference are interesting but liable to cause confusion to a newcomer to the subject, as they often make unprovable assertions based on "evidence" which in fact has no connection to the folk-plays; this is one of the reasons why the "pagan origins" fallacy has persisted so long out of its time.
Online, the best central source for reliable, up-to-date information is the Traditional Drama Research Group; Masato provided a link to their links page earlier on. A useful discussion of the origins of the Play(s) can be seen at Mystery History: The Origins of British Mummers' Plays; this is a short piece written in 1989 by Peter Millington of the TDRG, for the American Morris Newsletter.
I've never heard it suggested, as DMcG did earlier, that Come Write Me Down has roots in the Mummers' Plays, though I'd be interested to hear more about that. It was a popular 19th century broadside song (often titled Second thoughts are best), being to all appearances a re-write of an earlier broadside of the later 17th century (If you love me tell me so; Or, Loves fierce Dispute), which latter is rather earlier than the first known references to Mummers' Plays. Copies of both songs can be seen at Bodleian Library Broadside Ballads.