Sham el Nessim is an Egyptian national holiday celebrating the beginning of spring and Easter Monday. Today with the Islamisation of the country it is more of a national holiday than a religious holiday. It always falls on Orthodox Easter Monday. Although it's based on a Christian calendar date, it is celebrated by both Christians and Muslims in Egypt. It is a tradition dating back to ancient Egyptian feasts.
The name of the holiday comes from the Egyptian name of the Harvest Season, which means a day of creation, the smell of a breeze. According to Wikipedia, the Ancient Egyptians used to offer salted fish, lettuce, and onions to their deities on this day.
Even today this tradition continues but in a different way. Egyptians will eat salted fish, called fessikh with onions and shallots. At the front of each entry to the home some will slice part on an onion and leave it there. My mum does it until this day.
Feseekh with tahini, spring onions, lemon and lebanese bread
To celebrate the day, many spend the day having a picnic. This salted fish I speak of, namely feseekh is by far the most common tradition. It is considered a delicacy. It's a grey mullet that is dried, fermented and salted. It is marinated with salt, pepper, olive oil and other ingredients and usually stored in glass jars. It is has a very strong pungent smell. The fish is eaten with fresh lemon, shallots and tahini. People either love or hate it. I love it. Definitely a time of year I look forward to.