There is a custom to avoid marital relations on Rosh HaShanah, in keeping with the awesome nature of the holy days, and since these days are best spent in prayer and repentance. However, this is a custom, and not a halachic requirement, and therefore relations are usually permitted when mikveh night falls on Rosh HaShanah, and particularly if the couple has not yet fulfilled the mitzvah of Pru U'rvu (procreation), which entails having one boy and one girl.

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