Mentioning the fact that it is the first of the month, as well as the first of the year, in the prayers of Rosh HaShanah: Eruvin 39a, 40a-b
Mentioning, "Answer me today if today if the right day; otherwise, answer me on the other day": Eruvin 39a, 40a
Mentioning that it is also the first of the month in Shabbat prayers, if this is the case: Eruvin 40b
