What is the closing psalm at Chabad services about?

Question

On the seventh day of Pesach I went to Chabad for morning services. I've only been there a couple times before (always on a chag), so I'm not very familiar with their customs. At the end of each of the services I've been to, after Ein Keloheinu and Aleinu and Kaddish Yatom, there was a reading from Tehilim. When the rabbi announced it this time he said something like "today is Nisan 21, so the reading is (page number)". If I recall correctly, for that day it was Psalm 104.

This last part was not in their siddur; we used separate books of Tehilim. I don't know if that's just how it's done or if this means that what they did isn't normative -- I've only been to the one Chabad community.

What was that about? This wasn't the regular psalm for the day (Monday in that case); it sounded like this was something tied to date, not day of week, and anyway if it was day of week it wasn't the usual text.

(I hope to ask the rabbi about this at some point, though I don't see him that often. But I thought I might get a faster answer here.)

Answer 1

I believe that what you are referring to is part of the daily break-down of the Torah, Sefer Tanya and the book of Psalms that Lubavitchers refer to as Chitas (which stands, of course, for "Chumash, Tanya, Tehillim").

According to Hayom Yom for Nisan 21st (see here), the daily reading from Tehillim that day is Psalms 104-105.

Answer 2

There are different customs about reciting Tehillim (Psalms) daily. For example DailyTehillim.com does 1 chapter a day and complete the entire Sefer in 150 days. Some communities divide Tehillim throughout the week or month. Chabad custom is to recite Tehillim after morning services based on the monthly division.