Why must immersion be at night?

Tevilah may take place only after the seven clean days have been completely finished, so a woman may not immerse before nightfall on the seventh day.

The prohibition on daytime immersion on the eighth day or later is a rabbinic decree, lest a woman's daughter see her going during the eighth day and incorrectly conclude that daytime immersion is permitted on the seventh day. This decree applies to all women whether or not they have daughters. In extenuating circumstances (such as when the mikvah is in an unsafe neighborhood and going after nightfall is dangerous, or if there is a blizzard and it is dangerous to drive at night, etc.) there is room to permit daytime immersion on the eighth day or later. 

However, convenience is not considered a valid reason to override this rabbinic decree. Many mitzvot involve some level of inconvenience or even difficulty. We empathize with the difficulties you have as a working mother in finding time to do your preparations and immerse at night. You should make this mitzvah a priority, and try to work on the logistics to minimize the inconvenience. You can do the bulk of your preparations early in the morning before you go to work. Once you get home from work, you just need to shower, comb your hair, check your body for any chatzitzot, and immerse after nightfall. This whole process can be as quick as 20 minutes. If your husband is not around to help with the kids while you go to the mikvah, since you know a week in advance when you will immerse, you should use that time to schedule a babysitter. Alternatively, you can call the mikvah lady to see if you can schedule a late night appointment once your husband gets home.

We hope you are able to work out the logistics to immerse in a timely manner. We understand the resentment you feel towards mikvah since you are delaying immersion for days or even weeks. Please try to refocus and prioritize this mitzvah. Even if you find mikvah night stressful (and you are not alone in this regard), it is better to deal with it head on, right away, and not delay it, which causes even more frustration and resentment.

Please don't hesitate to be in touch for more suggestions or with any further questions.

B'Hatzlacha!