I guess Passover has started now, right? I'm not really that well versed in Jewish holidays, but from what I do know of them, they tend to have a greater sense of history than Christian ones. Christians often just seemed to say, "Okay, let's take this pagan holiday, throw Jesus in somewhere, and call it a day. Literally." While the primary Jewish holidays might not actually date back to Moses, they're definitely ancient. The Torah commands the observance of various festivals, including Sukkot, Shavuot, Rosh Hashanah, and Yom Kippur (the holiest day of the Jewish year). Passover has special connection with the Exodus, however, the name said to have come from the fact that the angel of God passed over the homes of the Hebrews when killing the first-born of Egypt. This was allegedly because the Hebrews smeared their doorways with lamb's blood, but since they were supposed to have lived in the Land of Goshen apart from the native Egyptians, I'm not sure why this was necessary. Couldn't God have just said, "Hey, skip over Goshen when on your killing rampage"? Regardless, the custom was to sacrifice and eat a lamb during Passover. Later, Christians introduced the idea that Jesus, who by all accounts was executed during Passover, was the new sacrificial lamb. But that's another story. Other foods eaten during the Seder include bitter herbs to symbolize the hardship of slavery in Egypt, charoset to represent mortar, and unleavened bread because the Hebrews left Egypt in such a hurry that they didn't have time to finish baking bread.
Now, I recently finished reading the book Who Wrote the Bible?, by Richard Elliott Friedman, and it makes a good case for how portions of the Torah were added or revised to support the points of view of the authors. The golden calf story is a good example. In Exodus 32, it's said that while Moses was chatting with God on Mount Sinai, his brother Aaron made an idol in the form of a young bull (translated "calf" because apparently English doesn't have as many words for the bovine life cycle as Hebrew does). If you read ahead to 2 Kings 10, you find more golden calves as symbols of God, this time placed in the cities of Bethel and Dan by King Jeroboam of Israel. Obviously, the more traditional priests weren't too fond of this, and it's thought that one of them might have composed the golden calf episode in Exodus as a response to this. It's sort of like saying, "See? We aren't the only ones who hate representing God this way. Moses himself hated it as well!" In addition, by making Aaron the wrongdoer in the piece and leaving Moses as innocent, it might well have been a way to promote the priests who traced their ancestry to Moses over those who claimed to be descendants of Aaron (although the Aaronid priests won out in the end). So why do I mention this? Partially just because I found it interesting, but also because it shows that the Torah probably doesn't all date back to Moses. Passover, however, does seem to be legitimately old. I have seen it suggested that it might have evolved from a more generic spring festival, but I guess that's really neither here nor there at this point. Interestingly, according to the Wikipedia article, "Pesach" might more accurately translate to "hover over," which casts some doubt on its getting its name from the angelic assassin.
One item that interested me about the Seder is how it's traditional to leave your door open and set a place for Elijah. Obviously that doesn't relate to the Exodus, since Elijah wasn't around until several centuries after that. So how did it come about? Well, part of it is that Elijah is said to have ascended bodily into Heaven, which presumably means he never really died. The book of Malachi ends with a prediction of his return on the "Day of the Lord," which came to be interpreted as meaning he'd show up back on Earth shortly before the arrival of the Messiah. Followers of Jesus identified John the Baptist with Elijah, since he more or less heralded the coming of Jesus (or at least that's the way it seemed, since Jesus' ministry began around the same time John was executed). People who think the Messiah has yet to arrive would probably say that Elijah hasn't yet returned to Earth either. So I would imagine that's why believers would expect to see Elijah and not, say, Moses, who's more closely associated with the festival but also is said to be dead and buried. There's also a tradition that Elijah will settle the difficult questions of Judaism when he arrives, including that of whether four or five glasses of wine should be served during the Seder. Since the question has never been officially resolved, the fifth cup is poured but not drunk, and said to be left for Elijah. This in turn developed into the idea that Elijah might actually show up to have a drink. If so, I hope he doesn't stop by too many houses. I don't even want to know what a guy who can bring down fire from the heavens is like when he's drunk.