What is the legal (Sharia) ruling regarding Sheikh Faisal Mawlawi's fatwa permitting the designation of a specific day for gathering for Qiyam al-Layl (night prayer) or Dhikr (remembrance of God), in which he cited evidence from the Qur'an, Sunnah, and sayings of scholars?

Gathering for remembrance (dhikr) is commendable, so long as it does not become a permanent habit. What is prohibited is the constant continuation of gathering in a specific manner. Assembling for the Qiyam (night vigil) prayer is also good, but without making it a regular habit or established Sunnah, because the Prophet (peace be upon him) did not consistently perform it in congregation except during Ramadan, nor did he designate a specific night for Qiyam. And whoever performs an action for which there is no command from the Prophet (peace be upon him), then it is rejected.