2,049. Delaying Immersion

Hilchos Issurei Biah 4:7

Let’s say that a woman delayed for many days and didn’t immerse. When she immerses, she should do so only at night. This is because if she immerses by day, an error might occur and another niddah may end up immersing herself on the seventh day (rather than on the eve of the eighth).

Hilchos Issurei Biah 4:8

If a woman fell ill, or if the mikvah was far away and women couldn’t reach it and return at night because of thieves, the cold or because they lock the city gates at night, then she may immerse by day on the eighth day or later.