Reb Elchonon's Question: Apparently Even Mitzvos Can Be Done During The Tosefes?

There is a rule that one is not allowed to get married on a Yom Tov because of the rule אין מערבין שמחה בשמחה [Moed Katan 8b]. The gemara in Ksubos [47a] says that there can be a case where a man gives his daughter away in holy matrimony on Yom Yov. Tosfos there is very bothered by this suggestion. How can a woman get married on Yom Yov - אין מערבין שמחה בשמחה and it is thus forbidden. [Tosfos knew about the gemara in Moed Katan. Back then there was not much else to do but learn:-)].

Answers Tosfos Oh-So-Famously: She got married during the time of Tosefes Yom Tov. When we add on to Yom Tov it is forbidden to do melacha as it would be on Yom Tov but the mitzva of simcha does not yet take effect.

Reb Elchonon [the one from Baranovitch and not the one from Givat Ze'ev:-)] in his Kovetz Shiurim questions this principle based on the opinion of the R"I of Korvil [Tosafos Psachim 99b] who says that all of the mitzvos of the night can be fulfilled during the time of Tosefes Yom Tov [if not for a special limmud that matza requires night]. Apparently then there is a mitzva of simcha even during Tosefes Yom Tov.

Usually I offer answers. This one I'll let you answer yourselves....