Mishnah Berurah 131:35 lists the following days on which Ashkenazim do not recite Lamnatzeiach (Tehillim 20) during weekday Shacharit between Ashrei and Uva LeTzion:
- Rosh Chodesh
- Erev Pesach
- Erev Yom Kippur
- Tish'ah BeAv
- Purim Katan
There is no mention of omitting Lamzatzeiach on Isru Chag.
However, I have been present in a number of Ashkenazi synagogues (mostly in Israel) where Lamazatzeiach was omitted on Isru Chag.
What is the source for this practice?