What is the ruling on calling for a collective fast and supplication, specified by a particular day, time, and supplication, for the victory of Muslims in Palestine or elsewhere, along with a request to disseminate this call?

It is not permissible to designate a specific day for fasting or to gather for collective supplication or recitation of adhkar with the intention of worship, as this is considered an innovation (bid'ah).

Neither the Prophet (peace be upon him) nor his Companions did so, and there is nothing in the Sunnah to indicate the permissibility of such gatherings and agreements. Rather, the Prophet (peace be upon him) prohibited singling out a specific night for worship over others.

It is permissible for a Muslim to supplicate for his brothers at any time and in any place, without designating a specific time or special fast for it. What is required is that a Muslim performs acts of obedience on his own initiative, without waiting for others.