One of the first things Moses did after leading the Hebrews (Israelites) out of Egypt was to instruct people to remember the Passover as a mark branded on their hand or forehead.
The first was about what they ate. After a week of eating bread without yeast:
Exodus 13:8-10 On the seventh day you must explain to your children, ‘I am celebrating what the Lord did for me when I left Egypt.’ This annual festival will be a visible sign to you, like a mark branded on your hand or your forehead. Let it remind you always to recite this teaching of the Lord: ‘With a strong hand, the Lord rescued you from Egypt.’ So observe the decree of this festival at the appointed time each year.
Then second was about sacrificing the firstborn. All firstborn males belonged to God. Animals were to be sacrificed; children were to be bought back. Child sacrifice wasn’t something God wanted, even if He laid claim to the children as His own:
Exodus 13:11-13 This is what you must do when the Lord fulfills the promise he swore to you and to your ancestors. When he gives you the land where the Canaanites now live, you must present all firstborn sons and firstborn male animals to the Lord, for they belong to him. A firstborn donkey may be bought back from the Lord by presenting a lamb or young goat in its place. But if you do not buy it back, you must break its neck. However, you must buy back every firstborn son.
And this ceremony was also a mark:
Exodus 13:16 This ceremony will be like a mark branded on your hand or your forehead. It is a reminder that the power of the Lord’s mighty hand brought us out of Egypt.