Another interpretation: “This month shall be for you” – this is analogous to a king who had treasuries filled with gold, silver, gems, and pearls. He had one son. As long as the son was young, his father kept everything. When the son grew and became an adult, his father said to him: ‘As long as you were young, I kept everything; now that you have become an adult, everything is given to you.’ So, the Holy One blessed be He kept everything, as it is stated: “And they will be for signs, and for festivals” (Genesis 1:14). When Israel arose, He gave them everything, as it is stated: “This month shall be for you.”
The King Who Waited for His Son to Grow Up
Shemot Rabbah 15:30