988. Beginning Hilchos Shvisas Asor - the Laws of Yom Kippur

Eiruvin 8:15

The conditional eiruv was made as follows: on Thursday, the one making the eiruv states, “If today is yom tov, my words are of no consequence. Otherwise, this should serve as an eiruv.” On Friday, he makes the eiruv again and says, "If today is yom tov, I made my eiruv yesterday and my words today are of no consequence. If yesterday was yom tov, this should serve as an eiruv." This only applies to the two days observed as yom tov outside of Israel. The two days of Rosh Hashana are treated as a single day, with the result that it is only possible to make an eiruv on the day before. [In Hilchos Shivasas Yom Tov chapter 6, the Rambam says that this does not work under our current, fixed calendar.]

Shvisas Asor 1:1

There is a positive mitzvah to refrain from acts of creative labor on the tenth day of the seventh month, as per Leviticus 16:31, “It will be a Sabbath of Sabbaths for you.” One who performs an act of labor fails to perform this obligation and also violates the prohibition of “You shall not perform any labor” (Numbers 29:7). If one performs labor intentionally on this day as an act of defiance against God, he is punished with kareis (spiritual excision). If one performs labor unintentionally, he must bring a sin offering of fixed value.