I am confused regarding the rules of when to omit E-l Erech Apayim (the paragraph before taking out the Torah) and when to omit Psalm 20 (said after the Torah is returned (Nusach Ashkenaz).
E-l Erech is said on Mon. & Thurs. only. But, I'm not sure if it is omitted on all days when Tachanun is omitted, such as today, Lag B'Omer, or not. My shul said it this morning. But, when Rosh Hodesh or Chanukah is on Mon. or Thurs. they don't. Are there different minhagim, here, or did my shul err, this morning?
Lamentze'ach (Psalm 20) - I know there is a list of omitted days in the siddur. But, it doesn't seem dependent on when Tachanun is omitted. We said it today. I don't think days like Tu B'Shevat or Pesach Sheni (Some omit Tachanun on Pesach Sheni) are omitted. But it's omitted on Rosh Hodesh, Purim and Chanukah and Tisha B'Av. What rules govern its omission?