מדרש אויף שמות 19:1

(Exodus 19:1) "On the third month of the exodus of the children of Israel from the land of Egypt": We are hereby apprised that we number the months from the exodus from Egypt.

(Exodus 19:1) "On this day they came to the desert of Sinai": It was Rosh Chodesh.

On the third month that Israel left the land of Egypt, on that very day they went to the desert of Sinai (Exodus 19:1) to receive the Torah, and on it the Holy Blessed One gave the Torah to Israel. And why didn't He give it to them when they were leaving Egypt? Because the Holy Blessed One said, "I will do good for them first, to demonstrate My might, and after that they will receive My Torah and My Commandments." I will explain a parable. What is this akin to? A king who wished to marry a woman. The king said, "I will do good for her, and after that I will marry her." Thus, the Holy Blessed One saw the congregation of Israel naked and clothed her, as it is said, "I dressed you... in linen... etc." (Ezekiel 16:10). He saw her barefoot and shod her, as it is said, "I sandalled you in dugong-skin." It came upon the sea, and He passed it though. He saw Amalek came upon her, and saved her. When Israel recognized the goodness and might of the Holy Blessed One, they all said, "All that the LORD has spoken we will do and hear!" (Exodus 24:7)