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

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

Taamei HaMinhagim, entry 128, says that we do not say Tachanun on the eve of Shabbat or Yom Tov (and other eves), as a reminder to pray the special prayers in that evening's prayers, or at least as a reminder that the special day that starts that evening.