1,061. Why the Previous Halachos No Longer Apply

Shvisas Yom Tov 6:13

The cases in halachos 6:11-12 only apply on the two days observed as yom tov outside of Israel. When it comes to the two days of Rosh Hashana, one who forgot to make an eiruv on Wednesday has no other chance to make an eiruv. He must either rely on another person who made an eiruv for him or transfer ownership of his ingredients to someone who made an eiruv. There is no other way for him to bake or cook for Shabbos. Similarly, if one did not separate terumah on a Wednesday that is the day before Rosh Hashana, he may not do so until after Shabbos.

Shvisas Yom Tov 6:14

The preceding ideas also only applied when the Sanhedrin would sanctify the months based on the testimony of witnesses. In those days, Jews who lived outside of Israel observed two days of yom tov out of doubt, as they didn’t know on which date the residents of Israel sanctified the month. Nowadays, the residents of Israel follow a fixed calendar and sanctify the months according to astronomical calculations. Therefore, the second day of yom tov is observed outside of Israel not from doubt but as an accepted practice. (This idea continues in the next halacha.)