The Jewish Calendar explained quickly

Jewish holidays occur on the same Jewish calendar date every year. The Jewish (or Hebrew) calendar has a different number of days than the common calendar because the Hebrew calendar is connected to the moon's cycles instead of the sun's. Therefore, the Jewish calendar has about 11 days fewer than the common calendar each year. As some Jewish holidays also mark certain yearly agricultural events and need to occur at specific seasons, the calendar makes up for the lag by adding a month every two or three years, at the end of winter. As a result, Jewish holidays don't always fall on the same day according to the normal calendar, but they always fall within the same month or two.

A Jewish "day" starts at sunset, because the first chapter of the Bible ends the account of the creation on each day with "it was evening and it was morning...". Therefore, as 2005 calendar says that Passover starts on April 24, families will be getting together for Passover dinner (the Seder) on the eve of April 23. The day will end after sundown on April 24th.