985. Making an Eiruv for the Second of Two Days

Eiruvin 8:9

In a case where an eiruv is needed for two days, if it is eaten on the first day, it is effective for that day but the one who made it may no longer make an eiruv in this manner for the second day. If one made an eiruv by walking to the designated spot on the first day, he may only make an eiruv for the second day by walking to that same spot, resolving that he is making this his base for that day. If one made an eiruv with bread on the first day, he may make an eiruv for the second day either by walking to the designated spot or by depositing the same loaf of bread that he used for the first day.

Eiruvin 8:10

When the calendar was determined by witnesses testifying about the appearance of the moon, it was possible for Yom Kippur to fall on Friday or Sunday. (This does not occur in our current calendar.) The Rambam is of the opinion that an adjacent Shabbos and Yom Kippur would be considered like one day and one continuous period of holiness (therefore requiring only a single eiruv for both days).