If I daven on 12-13 Tammuz in a Minyan that says Tachanun and I do not say Tachanun on these days, I just skip it, right?
Yes, you should skip Tachanun, but in a manner that doesn't call attention to your behavior. טעמים ומקורות: בכללות הענין ילה״ע מאג״ק ח״ז ע׳ קכה ואילך. והנה יש…