In the old Testament Passover was the twilight moment of the evening where the lamb was slain and it's blood painted on the doorposts to keep the people save from God's Spirit passing over Egypt.
"The LORD said to Moses and Aaron in the land of Egypt,“This month is to be your beginning of months; it will be your first month of the year. Tell the whole community of Israel, ‘In the tenth day of this month they each must take a lamb for themselves according to their families — a lamb for each household. If any household is too small for a lamb, the man and his next-door neighbor are to take a lamb according to the number of people — you will make your count for the lamb according to how much each one can eat. Your lamb must be perfect, a male, one year old; you may take it from the sheep or from the goats. You must care for it until the fourteenth day of this month, and then the whole community of Israel will kill it around sundown.They will take some of the blood and put it on the two side posts and top of the doorframe of the houses where they will eat it.", Exodus12:1-7
In the New Testament during that same twilight of that evening the slain Lamb of God and his blood saves the people from sin and God's Spirit can come over them.
"(Now it was the day of preparation for the Passover, about noon.) Pilate said to the Jewish leaders, “Look, here is your king!” Then they shouted out, “Away with him! Away with him! Crucify him!” Pilate asked, “Shall I crucify your king?” The high priests replied, “We have no king except Caesar!”.Then Pilate handed him over to them to be crucified", John19:14-16
The Evening Supper symbolizes this Sacrifice of Christ at the Cross. Christ couldn't keep this Evening Supper at Passover, because he was arrested, crucified and put in the tomb at the dusk of Passover. He is that Sacrificial Lamb of Passover.
"Then he took bread, and after giving thanks he broke it and gave it to them, saying, “This is my body which is given for you. Do this in remembrance of me.” And in the same way he took the cup after they had eaten, saying, “This cup that is poured out for you is the new covenant in my blood", Luke22:19-20
For Christians it is important to realize this, because when they keep the Lord's Supper it should be in the same dusk of Passover; to remember his sacrifice at the Cross every year.
How to determine Passover, that 14th day of the first month, during twilight? The New Moon falling closest to the Spring Equinox of the Northern part is the 1st day of that first month. That New Moon from sundown to sundown is the first day. The 2nd day from that sundown to sundown, 3rd day from that sundown to sundown, etc etc till you come to the 14th sundown to sundown 12 sundowns after the sundown of the New Moon of this 1st month called Abib in Spring. And the twilight at/after sundown the Passover is kept in which BAPTIZED* Christians keep the Lord's Supper.
The Lord's Supper actually is on the Feast Unleavened Bread so the bread and grapejuice used should be WITHOUT YEAST, so Unleavened Bread and unleavened wine meaning without alcohol so just some grapejuice!!
Before eating a small piece of the unleavened bread you should read Luke 22:19. And before drinking the wine you should read Luke 22:20. Make it a worthy Lord's Supper: don't eat or drink with appetite and wait for eachother to eat and drink.
1Corinthians11:27-29 says this:
"For every time you eat this bread and drink the cup, you proclaim the Lord’s death until he comes. For this reason, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord.
A person should examine himself first, and in this way let him eat the bread and drink of the cup. For the one who eats and drinks without careful regard for the body eats and drinks judgment against himself".
Like every special day of rememberance is done only once each year: also this Rememberance should be once each year on Passover. Any other way it would turn into a mockery!
Reading more about God's Days on God's Calendar: God's feasts pointing to God plan
God's Biblical Calendar on year (NL)