May a Kallah prior to her wedding go to Mikveh during the daytime?
If a Kallah prior to her wedding desires to immerse during the day time [for whatever reason she prefers] then she may do so even initially, so long as she has completed her Shiva Nekiyim and is immersing on her eighth day and onwards. Practically, while many Kallas immerse at night, it is also common for some Kallas to immerse during the day in order to have greater privacy and calmness in their first immersion.
Day of wedding-past Shiva Nekiyim: Some Poskim rule a Kallah may immerse even on the day of her wedding [if the Chuppah will take place at night]. Practically, A Kallah is not to schedule her immersion for the day of her wedding even if she is past the Shiva Nekiyim and the Chuppah will take place at night. She is rather to schedule the immersion for the previous night or another time prior. If for whatever reason she did not immerse prior to the day of her wedding then she may immerse on the day of the wedding, so long as the Chuppah will take place at night. Regarding if she may immerse on the day of her wedding even if it falls on her 7th day if it is, see the next Halacha!
 Rama 197/3; Shach 197/9; Maharil; Pardes Rimonim 197; Shulchan Hamareches Kiddushin p. 79; Nitei Gavriel Nida 1 4/2
 Shach ibid; unlike Rama ibid who implies to permit even on the 7th day, now although majority of the Poskim are lenient like the Rama, and so is the final ruling, nevertheless this only applies in a time of need, such as the 7th day is the day of her wedding. However when there is no time of need, certainly one is to be stringent like the Shach.
Other opinions: Some Poskim rule she may immerse even on the 7th day. [Implication of Rama ibid as bring all Achronim]
 Nitei Gavriel ibid footnote 3 that so is the custom in many places
 Rama 197/3 even regarding 7th day; Maharil; implication of Shach 197/9 regarding 8th day and onwards and so learns Minchas Yaakov, brought in Yad Efraim 197/3
 Degul Merivava 197/3; Pischeiy Teshuvah 197/10
 Yad Efraim 197/3
The reason: As the Shach 197/9 argues on the Rama 197/3 regarding the seventh day, and the same should apply regarding the 8th day unless it is a time of need. [Yad Efraim ibid]
 Yad Efraim ibid