Is what was done—the gathering of women in the mosque on the Day of 'Arafah after the 'Asr prayer for individual and collective dhikr and supplication, coinciding with the departure of the pilgrims—considered an innovation (bid'ah) or not? And what is its basis in Sharia?

Scholars have differed regarding thelawfulness of intending to go to mosques for supplication and remembrance on the Day of Arafah. Some permitted it, while others disliked it. Sheikh al-Islam Ibn Taymiyyah holds that the practice of "Ta'reef al-Amsar" (intending to go to mosques for supplication on the Day of Arafah) was performed by some Companions, and Imam Ahmad permitted it, while others, such as Ibrahim al-Nakha'i, Abu Hanifah, and Malik, disliked it, considering it an innovation. As for anything added to that, such as raising voices with supplications and uttering vain poetry, it is disliked.

As for intending to supplicate specifically at the time of the pilgrims' departure (nafrat al-Hujjaj), there is no basis for it, and it does not have any special virtue. Designating it for communal supplication and persisting in it raises concerns that it might be an additional innovation. It is more appropriate to abandon "Ta'reef" altogether to avoid disagreement, especially for women, for whom attending congregational prayers is not recommended.