The Jewish day
In the Jewish calendar, the day begins spans sunset to sunset. Our calendar shows the day of a holiday (or Shabbat). Thus if the first day of Purim is listed as Tuesday, the "Erev Purim" when we read the Megilla would occur on Monday night.
Reform holiday observance
The Reform movement celebrates one day for Rosh Hashanah, and 7 days for the holidays of Sukkot, Passover, and Shavuot. (The imported calendar shows Orthodox observances).