Daily Tehillim

I have the Tehillim Et Ratzon book and it is broken up by each day of the week. If I mostly say my Tehillim at night, I’m wondering if I should be going off of the lunar days or the solar days. For example, if it’s Saturday night after Shabbat, should I say Tehillim that are in the section for Sunday? Or should I be saying those on Sunday night even if after sundown?

Note: I’m new to tehillim but pledged to say one every day for a month in the honor of a fallen soldier. If you have any extra tips to add here I’m all ears.