Is it permissible to fast on a specific agreed-upon day, or to pray qiyam (night vigil) on a specific agreed-upon night?

Upon reviewing what was written in the aforementioned forum, we believe that agreeing to fast is a form of cooperation in righteousness and piety and enjoining the truth. However, several matters should be noted: Adhering to the Shari'ah by not fasting on prohibited days or singling out the night of Friday for prayer. If an agreement is made to fast on a day for which no specific merit has been mentioned in a text, it is not permissible to believe it has a special virtue. It is better that the agreement be to fast on virtuous days, such as Mondays, Thursdays, and the White Days (Ayyam al-Bid). The type of agreement must be clarified so that it is not obligatory, like a vow, but rather an encouragement to do good. And the discussion space should not be an arena for reporting on individuals; instead, worship should be based on concealment.