Q. How early can someone immerse in a mikva on Erev Rosh Hashana?

A. Kovetz Halochos (p. 50) maintains it can be done an hour before Chatzos (midday).

Rabbi A. Bartfeld as revised by, Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a