They had deep gratitude to God for bringing them out of Egypt. “Solemn observance.”
Dates of great miracles; do they mean anything to you? Have a culture of recording the great things God has done for you.
And it came to pass at the end of the four hundred and thirty years—on that very same day—it came to pass that all the armies of the Lord went out from the land of Egypt. It is a night of solemn observance to the Lord for bringing them out of the land of Egypt. This is that night of the Lord , a solemn observance for all the children of Israel throughout their generations.
Exodus 12:41-42 NKJV