Several Poskim, including the Hida (Rav Haim Yosef David Azulai, 1724-1806) and the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), mention that one should not read Tanach, including Tehillim, after Set Ha’kochavim (nightfall) on weeknights. On the night of Shabbat and Yom Tob, it is permissible to read Tanach.
There is some discussion among the Poskim concerning the status of Thursday night in this regard. The Arizal (Rav Yishak Luria of Safed, 1534-1572) mentioned a custom to read twenty-six Pesukim on Thursday nights, as written in the Hok Le’Yisrael text, and therefore some Poskim claim that once one is permitted to read these twenty-six verses, all reading of Tanach is permitted. Indeed, the Hesed La’alafim (Rav Eliezer Papo, 1786-1827) indicates that Thursday night is treated like Friday night in this respect. Therefore, those who wish to read Tehillim on Thursday night have a basis on which to rely. This is the ruling of the Ner Le’siyon.
Several Poskim observed that there was a custom to permit reading Tanach even on weeknights after Hasot (midnight as defined by Halacha). A number of major Poskim, including Rav Shalom Sharabi (1720-1777) and the Hida, questioned this practice, but the Ben Ish Hai writes that although it is preferable to study Torah She’be’al Peh after Hasot, the custom to allow reading Tanach has become widespread, and thus one may rely on this practice and read Tehillim after Hasot.
The Poskim also discuss the question of whether Tanach may be read on Mosa’eh Shabbat. The accepted Halacha is that since the sanctity of Shabbat extends until Hasot, one may read Tanach on Mosa’eh Shabbat until Hasot, and after Hasot there is in any event a custom to permit reading Tanach, as discussed. Hence, one may read Tehillim on Mosa’eh Shabbat. It thus turns out that this Halacha applies only to the four nights from Sunday night through Wednesday night.
Rosh Hodesh is no different from an ordinary weekday in this regard, and thus one should not read Tehillim on the night of Rosh Hodesh. We nevertheless recite the chapter of Borchi Nafshi at Arbit on the night of Rosh Hodesh, because once it was instituted as part of the prayer service, it may be recited, just as we recite Shir La’maalot Esa Enai every night at Arbit. Outside the framework of the formal prayer service, however, one should not read Tehillim on the night of Rosh Hodesh.
It must be emphasized that on the nights when Tehillim should not be read, it is counterproductive to do so. Often, text messages go out asking people to read Tehillim on behalf of an ill patient, and sometimes these messages are received at night. One should not read Tehillim before Hasot on the nights when it is improper to read Tanach.
The exception to this rule is somebody who is not capable of studying anything else besides Tanach. For such a person, it is certainly preferable to read Tehillim than to just waste time. Some Halachic authorities draw proof to this conclusion from the Gemara’s comment that when one returns home in the evening, "Im Ragil Li’krot Koreh Im Ragil Li’shnot Shoneh" – which means that if he is accustomed to reading Tanach he should read Tanach, and if he is accustomed to study Talmud he should study Talmud. The Gemara clearly allows learning Tanach at night if one is only accustomed reading Tanach, and is incapable of studying other material. Otherwise, however, one should ensure not to read Tanach on those nights when it is improper to do so.
Summary: It is improper to read Tehillim from nightfall until Hasot on Sunday night, Monday night, Tuesday night or Wednesday night, except on Yom Tob. Only one who is incapable of studying other material, and would thus otherwise waste time, may read Tehillim during these times. Even after Hasot, it is preferable to study Torah She’ba’al Peh than to read Tanach.
