What is the X in X-mas?
any verb whose past tense and past participle are not formed by adding -ed, -d, or -t to the present tense, a verb that does not follow the general rules of inflection
A verb in which the past tense is not formed by adding the usual -ed ending. Examples of irregular verbs are sing (past tense sang); feel (felt); and go (went). (Compare regular verb.)