The LORD's Passover is covered in detail in Exodus 12. The time each year is based off of the Spring-Solar-Equinox; which marks the beginning of the new year. 14 days from the Equinox, at sundown, marks the beginning of the Passover.
10 days out from the Passover: a male lamb, a yearling, without blemish, is to be separated from the flock, and killed on the 14th day - one lamb, for each household. The blood from this lamb was to be struck [strike it] on the two side post and on the upper door post of the houses, wherein they shall eat it.
For I [YHVH, Jehovah, I Am] will pass through the land of Egypt [system of bondage] this night .... and against the gods of Egypt I will execute judgment: I Am the LORD. And the blood shall be for a token upon the houses where ye are: and when I see the blood, I will Passover you .....
And this day shall be unto you for a memorial; and ye shall keep it a Feast to the LORD throughout your generations: ye shall keep it a Feast by Ordinance - forever.