IS ONE PERMITTED TO SAY TEHILLIM, PURIM AT NIGHT, EVEN BEFORE CHATZOS?(ACCORDING TO THE ONES THAT SAY IT SHOULD NOT BE SAID BEFORE CHATZOS, IS THE DAY OF PURIM DIFFERENT?)
The reason given by Kabbalists for not studying the Written Law at night is that the time is a time of din (strict judgment), which is inappropriate for the study of or recitation of Torah verses. See Shaar Hamitzvos (of Arizal, Parashas Va’eschanan); Yosef Ometz (of Chida, no. 54); and see Birkei Yosef (238:2) who brings a source from the Midrash concerning the distinction between night and day: day is a time for the Written Law, and night for the Oral Law.
However, Tzitz Eliezer (8:2, 17:3) writes that for times of need it is permitted to recite Tehillim at night, and it is customary to recite Tehillim on the night of Hoshanah Rabbah (Betzel Hachochmah 4:46). Mishnah Berurah (Shaar Hatzion 238:1) cites a dispute over the study of Scripture at night, and writes that there is certainly no prohibition, but only that it is better to study other parts of the Torah (the Oral Law) at night. This implies that wherever there is any need, it would be proper to recite Tehillim at night.
In view of the above, it can be suggested that because Purim is a particularly virtuous time for prayer, as the day on which our fate was overturned, and the day on which “whoever stretches out his hand, is given,” it would be permitted to recite Tehillim. Based on Chasam Sofer (Derashos 2:133), who mentions that the night of Purim is special in a similar context (reading the Megillah at night), it can also be suggested that on Purim din is turned to rachamim–though the particular question of Tehillim or Torah study is not mentioned.
According to those authorities who are more stringent with regard to Tehillim (mainly Sephardic authorities), there would not appear to be a formal difference between Purim and the rest of the year. Although Levush Mordechai (2:186) proves that the night of Shabbos is different to other nights, no such proof is brought concerning the night of Purim.