We bring in any period of forbidden work, early: Rosh HaShanah 9a
Adding Time to Shabbat
Source for adding time to the prohibition against Work: Rosh HaShanah 9a, Yoma 81b
Bringing in Shabbat early, and delaying its exit [that it shouldn't seem to be a burden]: Shabbat 118b, 119a; Pesachim 105b
Adding Time to Holidays
Source for adding time to the prohibition against Work: Rosh HaShanah 9a