Why are there two passover dates listed in the Old Tesatment?

I'm inclined to suppose that by "two Passover dates" you mean the two Passovers of Numbers 9:1-14 (see also 2 Chronicles 30:1ff.). The second Passover was authorized for those who were ceremonially unclean, and unable to observe the first. The Passover is, as you have indicated, a most important celebration. A friend of mine wrote about the Jewish holidays on the BSF Website.