Exodus 12:24, 25 And ye shall observe this thing for an ordinance to thee and to thy sons for ever. And it shall come to pass, when ye be come to the land which the Lord will give you, according as he hath promised, that ye shall keep this service.
After giving the instructions for the Passover, the children of Israel responded by observing the ordinance that night. And the children of God were to observe the ordinance for generations forever. And especially when they come into the land they would keep the service. Celebrating the Passover Feast annually was God’s ordained way of remembering what the Lord did in judgment and salvation; judgment for their spiritual enemies, salvation for His believing people.
The OT Passover Event pointed to and flowed into the NT Christ Event. Jesus is our Passover Lamb. The Lamb of God gave His life as a judgment on Sin, Death and Satan; and His death is salvation for the people, just as the Plagues were a judgment on the false gods of this world and the Red Sea Crossing was Deliverance for people. The correlation of the two Exodus Events is abundantly clear.
NT believers respond to this great salvation by observing the ordinance and keeping the service. Once a year we respond with the celebration of Good Friday and Easter. Once a week we respond by celebrating the Lord’s Supper in the Divine Service. Once a day we respond by remembering our Baptism in repentance and faith. We really do need to be reminded of God’s great salvation through the Gospel as often as possible because our sinful nature just keeps on being selfish.
Observing, keeping, thanking, worshipping, hearing, and believing is the healthy response. How often? Yearly, weekly, daily. When? When we come into the kingdom (land). How long? Forever.