1. If a vessel became Tamei on Yom Tov, one may immerse it on Yom Tov.
2. If a vessel became Tamei from a Rishon l'Tum'ah before Yom Tov, one may immerse it on Yom Tov.
3. The Gemara explains what a Nidah should do if she has no Tahor garments to wear after immersing on Yom Tov.
4. There is a dispute about why it is forbidden to immerse Tamei vessels on Yom Tov.
5. According to the letter of the law, people who are obligated to immerse may do so even on Yom Kippur or on Tish'ah b'Av.
A BIT MORE
1. This is because it is an uncommon case, and the Rabanan did not apply their decree against immersing vessels in an uncommon case (since all of the people are Tahor during the festival and are usually careful not to make vessels Tamei).
2. The Gemara explains that this is unlike the case of a vessel that became Tamei from an Av ha'Tum'ah before Yom Tov, which may not be immersed on Yom Tov.
3. She should immerse while dressed in order to take away the Nidah status in her clothes at the same time that she makes her body Tahor by immersing. She may do this despite the fact that she may not immerse her garments alone.
4. Rav Bibi: This is because we suspect that one will wait to immerse his vessels on Yom Tov, and he will mistakenly use them in the interim with produce of Terumah. Rava: It is similar to fixing a vessel (which is forbidden).
5. This allowance applies only when the person is clearly obligated to immerse on this day (see Tosfos).