What is the basis/source for the minhag not to say tachanun on 7 Adar? Is there any difference between Adar I and II?
Most communities recite Tachanun on 7 Adar. There are some kehillos that don’t say. This includes Belz Chassidim. As with many questions of Tachanun, there is no explicit source for this custom, rather a tradition handed down in that kehilla.
This applies to both Adar 1 and 2.