Mazal tov on your upcoming wedding!
In general, mikveh immersion is permissible only at night. Mikveh immersion during the day on day seven of the clean days could be invalidated by bleeding before nightfall and thus is prohibited. Immersion on day eight or later could give the impression that immersion is permissible on day seven during the day; it is thus prohibited by rabbinic decree except for special circumstances.
There is a special exception made for brides, who are permitted to immerse during the day. In Israel, common custom is for brides to immerse at night like other women. If you strongly prefer a daytime immersion prior to your wedding, you may be able to arrange this with the mikveh in advance.
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