23 Feb Purim Cuttin?
The Gemara in Megilah daf vov amud bais says that the fourteenth and fifteenth of the first Adar it is forbidden to eulogize and to fast.
The Rosh in Megilah in perek alef siman zayin says that this halacha applies only if Klal Yisroel already read the Megilah and in middle of the day Bais Din decided to make it a leap year, in this scenario one is not allowed to fast nor eulogize. In the event they did not read the megilah then one would be able to fast or eulogize someone.
The Tur (in siman taf resh tzadik zayin) brings the Rif, Rambam and the Smag that argue and say that on any given leap year it would be forbidden to fast or eulogize even in the event they did not read the megilah. The Shulchan Aruch brings both opinions and the Rema paskens like the Rif and those who agree with him.
We see from the Rosh that once klal Yisroel practiced Purim we don’t stop the celebration but not for everything. If they read the megilah and then decided to make that year a leap year one can’t fast nor give an eulogy, but on the other hand one does not give mishloach manor nor matanos lo’evyonim.
During Yom Tov sheini of Galiyos where a person from chutz le’Aretz is in Eretz Yisroel and in middle of the second day Yom Tov he comes to the conclusion that he will stay in Eretz Yisroel, The Teshuvos L’Horos Nosson chelek yud bais siman chof zayin brings in the name of the Chazon Ish that the day stops in the middle and all hilchos Yom Tov stop.
The question arises why in the case of Purim it does not get completely cut off whereas on Yom Tov it does not?
One could differentiate between Purim Kotton and Yom Tov. On Purim even if you will say that it gets cut down it will still have the name of Purim Kotton and therefore one would still be forbidden to fast or eulogize because it is still Purim Kotton. Yom Tov would be different because the minute it gets cut it is not considered Yom Tov at all to a person in Eretz Yisroel.