All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We have previously issued many Fataawa clarifying that in our view [in Islamweb] we consider that the most preponderant opinion is that it is forbidden for a woman in menstruation to enter the mosque and we gave evidences to support our view. So please refer to Fataawa: 85454 and 88170.
However, some scholars are of the opinion that it is permissible for a woman in menstruation to enter the mosque, among whom was Shaykh Al-Albaani. They [these scholars] argued that the chain of narrators of the narrations that were reported about forbidding a woman to enter the mosque in a state of menstruation is weak, and that there are more authentic narrations which suggest that it is permissible for her to enter the mosque when in menses. For instance, it is not reported that the Prophet ordered the black woman, who used to sleep in the mosque and clean it, to leave the mosque when she was in menstruation. Moreover, the Prophet said: "A believer does not become impure." The Prophet also told 'Aa'ishah : "Pass me the rug [from her room] into the mosque [through the window or through the door]."
Allaah knows best.