When is the seder meal celebrated?

The "seder meal" is observed on the first evening of Pesach (Passover), which occurs on the 15th of Nissan according to the Jewish calendar. This places Passover somewhere between late March and the middle of April according to the Gregorian calendar.

[The Jewish calendar is made up of alternating 29 and 30 day months, corresponding to the appearance of the new moon, with a "leap month" added every 2-3 years to synchronize with the solar year.]

Orthodox Jews outside of Israel observe the "seder meal" on the first and second evenings of Pesach, because there was doubt as to the precise date in former times.