What is the best night for performing Qiyam al-Layl weekly or monthly for someone who is rarely able to observe it regularly?

There is no specific night that is recommended to be singled out for worship. Rather, a Muslim prays whenever it is easy for him and he feels energetic. However, singling out the night of Friday for night vigil prayer (qiyam) must be avoided, as there is an explicit prohibition against it, based on the Prophet's (peace be upon him) saying: "Do not single out the night of Friday for night vigil prayer among all other nights." Nevertheless, it is permissible to perform night vigil prayer on Friday night or to fast on Friday if it is not done with the intention of singling it out, but rather because it is a day of leisure or because it coincides with a day whose fasting is recommended.