פירוש על בבא מציעא 118:10

Imma Shalom: That was her name.

A full [month] and a deficient [month]: She reasoned that it would be a deficient month and the ratification [of the incoming month] would be on the thirtieth day, so that he would not lower his head. But it was full and they did not ratify it until the thirty-first day; so she was not careful about him on the thirtieth day and he lowered his head.

A certain day [was the day of] the New Moon, etc.: Rashi explained, "She reasoned that it would be a deficient month and the ratification [of the incoming month] would be on the thirtieth day, so that he would not lower, etc." And this was possible in their time when they would sanctify [the New Moon] by a sighting, so they would not also treat the thirtieth day in a full month [as] the day of the New Moon, but rather only the thirty-first day. But we are accustomed also in a full month to treat the thirtieth day [as] the New Moon as well. And it is a little difficult according to his explanation - since, "A certain day was the day of the New Moon," implies that it was truly the New Moon on that day. So the [correct] explanation appears to be the opposite: That that thirtieth day was the New Moon, and she mixed up the full month and the deficient month. As she reasoned that it would be a full month and it would also be the New Moon on the next day on the thirty-first day, so he would not lower [his head]. But the month was deficient, so he did lower his head on the thirty-first day. And if you will be exacting, you will find it simple.