How many days of slichos

Dear Rabbi, I’d like to understand, why, according to the custom of the Ashkenazim, the number of days in which we say slichos varies, and every year there is a different number of days of slichos. How does one determine, how many days of slichos there are?

Dear …

The rule of thumb to figure out how many days Ashkenazim say slichos is this: there has to be at least four days of slichos before Rosh Hashana. The start is always on Saturday night. Therefore, if Rosh Hashana falls on Thursday or Saturday, slichos begin on the Saturday night closest to Rosh Hashana. But if Rosh Hashana falls on Monday or Tuesday, slichos begin on the Saturday night of the previous week.

There are two reasons given for the requirement to say slichos for at least four days. One reason is akin to the offering that needs to be inspected for blemishes four days before it is offered, and so before we approach judgment on Rosh Hashana, we inspect and prepare ourselves for four days. Another reason is that there is an interest in fasting ten days between Rosh Hashana and Yom Kippur, but during those days, there are four days when fasting is not possible: the two days of Rosh Hashana, Shabbat, and the eve of Yom Kippur. Therefore, these are compensated by the four days before Rosh Hashana.

The reason for always starting on Saturday night is to establish a fixed time for the beginning of the recitation.

Mishna Berurah, ch. 581, subsection 6