Women entering mosque in Janabah
Janabah = menses
Allah the Almighty says:
"O you who believe! Do not approach the prayer while you are intoxicated, so that you would know what you are saying, nor in a state of janabah, except if you are passing though, until you have perfromed ghusl..."
[Sura an-Nisa (4): 43]
Some say that prayer means 'place of prayer' or mosque, and that someone in the state of janabah may only pass through a mosque without sitting for a long time.
The correct meaning of this verse is that it refers to those who become in a state of janabah while they are passing through (i.e. travelling). Such individuals may not pray until they have performed ghusl or in the absence of water, tayammum.
[Tafsir Ibn Kathir, Tafsir al-Qurtubi]
Some cite the following hadith for prohibiting women entering mosques:
Prophet Muhammad said, I do not allow entering the mosque for a junub or a menstruating woman
However this hadith is classified as unauthentic because of Jasurah and other weak narrators in its chain of narration.
[al-Albani's, Irwa ul Ghalil, no 193]