Why don't we say Tachanun at Mincha on Erev Shabbat or Erev Yom Tov?

Question

It says in the Rambam that there is a custom not to say Tachanun at Mincha on Erev Shabbos and Yom Tov. What is the reason for this custom?

Answer 1

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We do not say Tachanun on Shabbos and Yom Tov which are "Yemei Simcha" days of Simcha. Since Mincha prior to these days we are already in the Simcha mode, we do not say Tachanun either.