http://revach.net/article.php?id=745

Section: Halacha   Category: Tshuvos
Btzel HaChochma: Giving Tzedaka At Night

It is written in the name of the Arizal that we don't give tzedoka at night. If so asks the Btzel HaChochma (5:143), it is a Mitzvas Aseh SheHazman Grama, a time dependent mitzva and women should be exempt. Yet we see that the Chinuch says both women and men have the mitzva of tzedoka. Why did the Arizal say not to give tzedoka at night?

Nighttime is a time of Din and since tzedoka is an act of Chesed its appropriate time is during the day. Therefore explains the Btzel HaChochma, since the Shulchan Aruch clearly states the amount one must give for tzedoka it is better to give it during the day because that is the proper time. If you need to give tzedoka at night it is preferable to set aside the money by day even if the poor person can only take it at night. Nevertheless if a poor person asks for money at night you are certainly obligated to give him and are not exempt even according to the Arizal.