Sham El-Nessim is an old Egyptian tradition which can be traced back to an ancient festival welcoming the new harvest. It is celebrated by eating salty fish called "Herring" and vegetables.

And the LORD said to Moses, "Make a fiery serpent, and set it on a pole; and every one who is bitten, when he sees it, shall live." So Moses made a bronze serpent... Numbers 21:8-9 ...