Immersion Before Entering the Temple, or Before Yom Tov [Holidays]
Before Yom Tov
Requirement of Purification before Holidays: Rosh HaShanah 16b
Considering people pure during the holiday, on the assumption that they immersed beforehand: Beitzah 11b
Extending the assumption of purity to products that were touched by people on Yom Tov, even when the products are now being used after Yom Tov: Beitzah 11b