As the young woman matures physically and enters puberty she will begin to menstruate. At this life-milestone which begins a more mature phase of her womanhood, she becomes "niddah"--off-limits. Passionate physical contact by a male is expressly forbidden until after the couple commits to one another by marrying. This prohibition applies even to an "engaged" couple. The sexual relationship only becomes permitted after the woman has immersed properly, following the appropriate checks, and subsequent to the halachic wedding ceremony--the qiddushin followed by the huppah.
The engaged couple must remain very vigilant in observing the laws prohibiting sexual contact during this period. It is ultimately for the good of the couple to resist the temptation and remain clear-headedly sure about their commitment to one another without the confusion that a sexual relationship would bring at this point. Of course the man is allowed to look at the normally uncovered parts of the woman he intends to marry in order to be sure that he is attracted to her.
The wedding date is chosen taking into account the woman's menstrual period in the following way. The woman should become aware of the distinctive aspects of her menstrual period by observing when she begins to get her period and how long her menstrual bleeding normally lasts. An average period is 30 days from the onset of menstruation to the next onset of menstruation. It is advisable to choose a date close to the end of the thirty day period (about 23 days), so as to ensure she will not be niddah during her wedding, but only soon afterwards.