Rosh Hashana marks the conception (when the thought of creation arose within the mind of G-d) and Pesach marks the actual birth (when an actual “creature” evolved that could hold the level of consciousness, called yechida, the capacity to understand what it means that G-d is one on the 50th gate level of understanding). That “creature” is Kenesset Yisrael, the Mystical Body of Israel, born on the other side of the parted sea that served as its birth canal.