Tonight at sundown is the end of the day of Passover and the First Day of Unleavened Bread. It is a holy day ordained by God (Lev 23:6-7). There is a “holy convocation” on the first day.
However, some groups will only have one service during the day tomorrow. Most COG organizations will hold a special memorial tonight about sundown in small groups. The memorial is more informal than a church service and is usually accompanied by a special meal with brethren. The focus is not on the meal, but rather on what God has done for us by bringing us out of spiritual Egypt.
37And the children of Israel journeyed from Rameses to Succoth, about six hundred thousand on foot that were men, beside children.
38And a mixed multitude went up also with them; and flocks, and herds, even very much cattle.
39And they baked unleavened cakes of the dough which they brought forth out of Egypt, for it was not leavened; because they were thrust out of Egypt, and could not tarry, neither had they prepared for themselves any victual.
40Now the sojourning of the children of Israel, who dwelt in Egypt, was four hundred and thirty years.
41And it came to pass at the end of the four hundred and thirty years, even the selfsame day it came to pass, that all the hosts of the LORD went out from the land of Egypt.
42It is a night to be much observed unto the LORD for bringing them out from the land of Egypt: this is that night of the LORD to be observed of all the children of Israel in their generations. (Exodus 12:37-42, King James Version)
While this isn’t listed in Leviticus 23, it is still an observance instituted by God. Today, the COG uses the occasion to reminisce about how Jesus broke our bonds of spiritual slavery and set us free from our sins.