Seems odd that a people would make up a religious holiday to escaping from slavery from another country. If you're going to invent a holiday it seems you'd invent one where your people are great conquerors not slaves.
From Wiki on Passover:
Passover... is a Jewish and Samaritan holy day and festival commemorating the Hebrews' escape from enslavement in Egypt.
http://en.wikipedia.org/wiki/Passover
Yeah, you'd think so, wouldn't you?
Since we're just chewing the fat here, let me spin a little (ok, big) yarn. It seems that back about 2500 BCE or so, a bunch of primitive herdsmen in an area on the eastern Med decided to take up newfangled agriculture, noting that their friends and acquaintances in neighboring states seemed to do well that way. Just keeping up with the Sumerians, you might say. The trouble with this, as well as the glory, is that you don't have to keep pulling up stakes and chasing wild animals across the plains and hills. You can set a regular schedule, usually. Unfortunately, when you do that, others know where to find you, sometimes with intent to pillage, rape and burn, so they settled in cities with some defensive walls and such.
The trouble with that is that you need someone to organize things, to tell the farmers where their plots are, and when to go out and plant, and how to defend the walls. No, not everyone can be the landowner or a spearman on the walls, someone has to sow and reap, clean up the wounded and bury the dead. Classes were born, at least in this corner of the sand box.
That is also a problem. Because some people want to be spear-chuckers or wear the golden torqs or whatever, and many don't want to get their hands icky or sweat in the sun. Some don't want to push the stones to make the tax collector's fine, new house. Particularly when the tax collector is a *****. Revolutions seem to start pretty easily in these climates, and so it seems to have been in our land of milk and honey.
In 2200 BCE, the city of Ai was razed, but there is no sign of external siege, nor any known armies wandering around at the time. Jericho suffered the same in 1500 BCE, and Hazor in 1200 BCE. It seems that when the masters get a bit crazed with monumental works, the people of the East Med have little patience. Jews can be funny that way.
In 1200 BCE there were 25 cites in Canaan, holding perhaps 3-5 thousand people together. By 1000 BCE, there are 250 cities, with a total population of 45,000. No sign of any exterior assault or immigration, only interior rebellion and population growth once the big cities were (mostly) abandoned. Same basic pottery types, but with new decorations, and the houses are on a new common plan. Also, no kings. It took a few years to get around that, but they say David had the Look, you know?
So perhaps these Israelites, as they became known, had some sense of their journey - away from tyranny, into a new sort of egalitarianism. How does one establish that fact in perpetuity to later generations? Why, by inventing a story, and later on even writing it down - damn the libruls and their expensive ideas. You want the history preserved, but you want to make it a lesson on how to do things, hard won truths learned along the way. So you develop a script, drill it into some unruly kids heads, and set them to writing it all down. But what do you write? Certainly something entertaining, but full of wisdom, natch. The Great Jewish Novel.
On the north wall of Karnak, Seti I, father of Ramsses II, wrote about conquering the Shasu (actually, wrote about slashing them, like in college football), who lived in Midian, in roughly the Negev desert. They called themselves in their barbaric script, YHW (Yahoo - who knew?), and incidentally the name of their corporate god. So he was there, in Israel, along with Baal, Ellil, Asherah, El and the whole happy deitic group.
So how do you make a good, literary clean break with the past and inspire your people to carry on in the new way? You invent a new corporate religion. At least, that's how L. Ron did it, and how can you argue with success (so far)?
Inside the super-sekrit Iursidictional Council of new Religion for the masses Institute (in proto-Hebrew, "INRI", they hadn't gotten rid of the vowels, yet, that's next week):
Let's see. We decided last week to settle on one god, and lay the rest off, those old city patron gods have to go anyway. That'll shake the traditionalists up. What else? Well, I have a promising tale about the real beginning, that great Uncle Shimsam heard when he was traveling in Ugarit (Note to self: Don't forget to edit out all those other gods in there). We need to explain how it looks like were going to hell in a hand basket, but how it will all turn out OK in the end if they just have faith and keep the gerbil wheel turning. There's a neat story about a flood over in Ur (old hat to the Egyptians, they have floods all the time). We can dress that up some. Speaking of which, we need a heavy to inspire cohesion, you know? No one heavier than the Egyptians and all that Book of the Dead stuff. Ho ho, snigger. And a hero - um, call him Moe. That probably even means something in Egyptian, too. If we scrape enough of these Just-So stories together, we don't have to admit where we got any one of 'em. Just keep that mohel out of this, pul-ease. --What do you mean, overruled? --
Ahem. Anyway, blame all this stuff on Moe, he's got the fancy staff and the tablets and all. Might as well. If this balloon doesn't fly, I don't want them searching my temple for anything.
One more thought - tack all those damned rules in here too. Might as well, it will make them sorta official. And don't complain about writing cramps. You know this is only gonna get thicker - all these super-romances do. And just think, you'll hold the Copyright©.
So let it be written, so let it be done.
Oops, wait - one thing I thought of. We gotta have some holidays. Let's see, release from Egypt sounds likely, particularly for slaves (yeah, I know - just don't tell them that). When do we want it? No, March 15th is booked; they're not fit to talk to until April. Feb's taken, Hallmark would sue something fierce. Ummmmm... What if we make it the, uhhhh,
Code:
Rosh Hashanah (who's he? Oh, nevermind - just follow) will
fall on September M, where
N + fraction = (floor(Y/100) - floor(Y/400) - 2) +
765433/492480 x ((12 x G) mod 19) +
(Y mod 4)/4 - (313 x Y + 89081)/98496
M = N + postponement rule below
1. If the day calculated above is a Sunday, Wednesday,
or Friday, Rosh Hashanah falls on the next day
(Monday, Thursday or Saturday, respectively).
2. If the calculated day is a Monday, and if the
fraction is greater than or equal to 23269/25920
(0.898), and if (12 x G) mod 19 is greater than 11,
Rosh Hashanah falls on the next day, a Tuesday.
3. If it is a Tuesday, and if the fraction is greater
than or equal to 1367/2160 (0.633), and if
(12 x G) mod 19 is greater than 6, Rosh Hashanah
falls two days later, on Thursday.[FONT=verdana]
[/FONT]
The previous Pesach (that's what I thought to call it) is
M days after March 21st.
Even better, we don't tell anyone how to figure it out. Yes, we can do that
in a top sekrit meeting.That ought to keep us in beer for a couple of
centuries, as long as I care about, anyway.
OK, then, let's see who needs to do what.....
Does that help?
. (Heads off for the Penitent-Post Corner to meditate on existential sins)
And yet DOC quoted the same long post to reply with a one-liner immediately after I (politely I hope) admonished you... not surprising since DOC doesn't listen to anyone other than himself or people with embellished credentials. 
