Most people don't think of Passover as incomplete. Usually, it's quite the contrary. After eight days of (matzo) brei and balls, Chag Hamatzot (the Holiday of Matzo) often feels like it's overstayed its welcome. Bring on the leaven.
But Passover is actually incomplete, technically speaking. Its time is extended by seven weeks: the Omer, so named because of the portion of wheat that was set aside to count the 49 days before Shavuot, the completion of this week or weeks -- a holiday whose name itself is "Weeks." Shavuot is also called Atzeret, meaning "Assembly," like Shemini Atzeret, the day of assembly on the eighth day of Sukkot. In both cases, the Day of Assembly marks the conclusion and culmination of the holiday. In the fall, that means celebrating the renewal of the cycle of reading Torah. In the spring, it means celebrating the receiving of the Ten Commandments: the Children of Israel are redeemed from slavery, they cross the Red Sea and now, literally, the Covenant is sealed.
Only it isn't sealed right away. There's some defect, something lacking in the Israelites, even after the exodus from Egypt. Passover leaves us hanging. The plagues happen, the Israelites cross the sea, Miriam dances and then what? Then there's a delay. In fact, the Omer provides what Passover lacked: time. And there are at least three aspects to this phenomenon.
First, the delay between Redemption and Revelation signifies the incompleteness of the "peak experience" of the Exodus, since time is what differentiates the ethical from the aesthetic. If there were no time, there would be no ethics; there could only be the aesthetic fitting together of the components of the universe. Without action, which requires time, the concept of the ethical is meaningless, for the ethical is a category whose coherence depends on the temporality of action.
Obviously, Passover's tale is action-packed. But I think it is primarily an aesthetic moment, as DeMille and Spielberg have shown us. The slaves are set free, while the arrogant are defeated by their own arrogance, as the weighty chariots of their technology are outdone by a low tide. But from the Israelites' point of view, it is primarily something that happens to them -- not something they do. They are observers of most of the story, not participants. It's a show, an experience, an aesthetic moment of wonder. They don't even hesitate when it's time to leave Egypt; they get out and run, which is why the matzo is so flat and unleavened. So time is needed to complete the aesthetic with the ethical.
Second, for the revelation at Sinai to come right after the Red Sea would make it inauthentic. It would be just another "Wow," a peak experience like those which regularly take place in spiritual practice: beautiful moments, immediate promises made and then, after a few days or maybe weeks or months, the enthusiasm passes and so do the promises. It takes time for peak experiences to unravel. The ecstasy lasts for awhile before the laundry kicks in -- and, like the Israelites with their Golden Calf, we crave more of it. In this way, the Omer is not unlike sleeping off the hangover before you propose to that beautiful boy or girl you met at the party. See how they look in the morning, and more importantly, how you feel. f you want to seal the covenant at Mount Sinai, wait until the headache subsides.
Third, the cultivation of ethical consciousness takes time. In one Kabbalistic interpretation, each of the 49 days of the Omer was a particular type of moral purification. To this day, you can buy "Omer guides" of this type. The idea is that through the time of the Omer, particular moral dispositions can individually be "repaired" or purified or improved or contemplated. It takes time to do so. Ethics takes time to ripen. This is true internally and externally. When you love someone, you don't love them for an instant of aesthetic wonder. You love them from the pattern of beauty that emerges from their acts, from the things they do for you and you do for them, from the way you feel when you are speaking with one another (instead of speaking to, or talking at). Whether internally as contemplation or externally as relationship, ethical consciousness requires time to unfold.
Of course, sometimes it takes more than seven literal weeks to reach seven figurative weeks, to reach that completion of understanding necessary to receive the revelation. But, as they say, one day at a time.