All Jewish holidays and observances, with the exception of minor fasts, begin on the evening prior to the first day. Almost alls of the holidays require special additional prayers and services that are held in the synagogue.
Jewish holidays most often prescribe restrictions as are done on Shabbat. Business, school and secular activities cease a few hours before sundown to allow for holiday preparation and the actual holiday restrictions go into effect approximately half an hour before sundown on the eve of the holiday. Some of the restricted activities include all forms of vehicular travel, writing, direct use of electrical devices, measurements and any preparations for, or discussion of, normal weekday activities or responsibilities. Restrictions end approximately one hour after sundown of the last day and cannot be relaxed over the course of the holiday except in circumstances when there is mortal danger to human life.
Fast days include special prayers and extended synagogue services both in the morning and afternoon. Food and water are forbidden until approximately one hour after sunset.
Sources: Orthodox Union