CO the reason there is no mentioning of Easter or Christmas is God did see the need for either one, no authority from God to observe them. It is of man...
What is to be observed is the Lords supper in rememberence of Christ death where he shed His blood for the redemption of man kind.
(Heb 10:4). They merely prefigured or pointed to the coming of Christ who would be the perfect and final atonement for sin, "Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us." (Heb 9:12) In Heb 9:22 we read "And almost all things are by the law purged with blood; and without the shedding of blood is no remission.