What happens in the case of a bride menstruating on her wedding day or her menstruation ending a few days before her wedding day and cannot go to the mikvah?
Does the rabbi refuse to perform the chuppah? What are the options?
Practically speaking, the wedding goes ahead as planned, with minor differences at the ceremony:
But nobody would notice, if they're not looking carefully.
Once they get home it gets more complicated.
The Kitzur Shulchan Aruch in סימן קנז - דיני כלה הנכנסת לחפה elaborates:
סעיף ג': יֵשׁ לִזָּהֵר שֶׁלֹּא תִנָּשֵׂא אִשָּׁה עַד שֶׁתִּטְהַר מִטֻּמְאָתָה וְאִם הָעֵת דְּחוּקָה וּצְרִיכִין לַעֲשׂוֹת הַנִּשׂוּאִין בְּעוֹדָהּ בְּטֻמְאָתָה, אוֹ אִם אֵרַע שֶפֵּרְסָה נִדָּה לְאַחַר הַחֻפָּה קֹדֶם שֶׁנִּבְעֲלָה, לֹא יִתְיַחֲדוּ בְּלִי שְׁמִירָה עַד שֶׁתִּטְבֹּל. וְנוֹהֲגִין לִקַּח קָטֹן אֵצֶל הֶחָתָן וּקְטַנָּה אֵצֶל הַכַּלָּה, וְאֵין מִתְיַחֲדִין אֲפִלּוּ בַיוֹם בְּלֹא קָטֹן אוֹ קְטַנָּה. וּצְרִיכִין שֶׁיִהְיוּ גְדוֹלִים קְצָת שֶיּוֹדְעִין עִנְיְנֵי בִיאָה, וְלֹא גְדוֹלָה מַמָּשׁ שֶׁכְּבָר לָבְשָׁה יִצְרָהּ שֶׁיֵשׁ לָחוּשׁ שֶׁתִּתְפַּתֶּה, אֶלָּא בֵּינוֹנִים. וְאֵין חִלּוּק בָּזֶה בֵּין בָּחוּר לְאַלְמָן וּבֵין בְּתוּלָה לְאַלְמָנָה, שֶׁכֹּל שֶלֹּא בָעַל אִשָּׁה זוֹ מִיָמָיו, חָיְשֵׁינָן דְתַקִיף יִצְרֵהּ, וַאֲסוּרִין לְהִתְיַחֵד בְּלִי שְׁמִרָה.
One should be careful to not marry the couple until she has gone to the Mikve and is no longer a Niddah.
In case of great need, if they need to go ahead before she goes to Mikveh, or if she becomes a Niddah after the ceremony but before they have lived together, then they may not be secluded - even during daytime - without another person present, until she goes to the Mikveh.
The custom is (that they sleep in separate rooms and) a boy sleeps in the groom's room and a girl with the bride.
The boy and girl must be old enough to understand intimacy (and blabber about it, should it happen), but young enough not to be interested, so that they cannot be persuaded to play along. (My guess: between 6 and 12)
These rules apply whether the bride and/or groom have been married before or not; as long as they haven't been married to each other.
She has a chuppas niddah. It was a dispute among the Rishonim as to what extent chuppas niddah is effective, but common practice is that they go ahead with the wedding as normal (Shach Yoreh De'ah 192:8).
There are some questions that arise in this situation. For example, does the groom put the ring onto the bride's finger? There are varying customs. In my community it was taught that in the event of a chuppas niddah to put the ring on as normal and just be careful not to touch. Some communities would just pass the ring in such a situation.
Also, since the couple cannot consummate the marriage, and has never been together before, there are yichud issues, and they may not sleep together alone in the same room (Shulchan Aruch Yoreh De'ah 192:4). Similarly, they should "hide" someone in the yichud room after the chuppah, because real yichud is not permitted at that point. (The hiding avoids revealing to everyone that she is a niddah, which some people find uncomfortable.)
Generally prevention is better than cure, nowadays there are tablets a woman can obtain from her doctor which will delay her periods - a combined progesterone oestrogen pill. The woman ideally should know when the onset of her menses will occur, if this coincides with her wedding, it may be prudent to take the pills... Otherwise all the aforementioned is in play.