We understand that Friday night mikveh can be complicated, and you would prefer to be able to start your five-day minimum on Sunday. Nevertheless, if you are not bleeding or spotting at all, you should not simply declare yourself niddah.
However, it is permissible to do an internal bedikah on the day you hope to become niddah. If you find even a tiny amount of blood on the bedikah cloth – which wouldn't be unusual a day before you expect your period – you would be niddah and you would be able to start your five-day minimum that day.
Whether this is a good idea depends on your circumstances. If you aren't certain that you’ll have niddah bleeding on Monday, or that you would be able to get a successful hefsek taharah on the fourth day of your period, than this could lead to your spending extra time in niddah for no reason. Perhaps you could find a way to work things out so that a Friday night immersion would go more smoothly (please feel free to write to us if we can help with that). Otherwise, if you and your husband both agree, it would be permissible to delay immersion to Saturday night.
On the other hand, if you're very certain that bleeding will start Monday, and a Friday night immersion would be unavoidably difficult, this strategy might be worthwhile for you.
If you're using combination hormonal contraceptives, and it works out that you usually get your period on Monday and that mikveh night comes out on Friday, you can shift your cycle by a few days for more convenient scheduling.
Even if you find that you naturally wind up with Friday night immersions often (e.g., you have a very regular four-week cycle), you might consider consulting your physician about hormonally manipulating your cycle a bit one time to shift your pattern.
Please write back with any further questions.