The Hebrew Calendar

B'not and B'nei Mitzvah's age is calculated according to the Hebrew Calendar

A Jewish boy automatically becomes a Bar Mitzvah after reaching the age of 13 years; a girl automatically becomes a Bat Mitzvah after the age of 12 years.

The 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 Hebrew 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 common 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 started on April 24, families gathered together for Passover dinner (the Seder) on the eve of April 23. The day ended after sundown on April 24th.