The members of our iWorship listserve raised a very important question recently:

This year, the first Passover Seder falls on a Saturday night - what changes (if any) should be made to the Seder? Should there be a Havdallah ceremony? If so, at what point in the Seder?

All good questions!

Here is the traditional answer:

If the Seder falls on Motzaei Shabbat (Saturday evening), Havdallah should be observed to separate the Sabbath from Passover (Yom Tov). The Havdallah ceremony will take place at the time of Kiddush over the 1st cup of wine (after lighting the  Yom Tov  candles). The blessing should be recited over the wine, and the usual Kadesh paragraph found in the Haggadah following Borei Pri Hagafen should be recited before continuing on with the other Havdallah blessings. The blessing over the spices is omitted so the next blessing after Kadesh will be the blessing over the fire (the Yom Tov candles may be used in place of a Havdallah candle). Following that is Hamavdil ben Kodesh and the Shehecheyanu. 

Shabbat affects the preparations for Passover in other ways as well.  Traditionally, all cleaning and burning of chametz is done before Shabbat begins. As well, the Fast of the Firstborn Child (traditionally observed by firstborn sons on the day leading up to Passover) is observed on the preceding Thursday, rather than on Shabbat.

For further reading, check out these books:
Pesach: Passover - Its Observance Laws and Significance, Rabbi Nosson Scherman & Rabbi Meir Zlotowitz
Every Person's Guide to Passover, by Ronald H. Issacs