Ashkenazim say a short paragraph starting אל ארך אפים on most Mondays and Thursdays before reading from the Torah.
It is a request to Hashem not to rebuke us in His anger and to give us forgiveness. It is reminiscent of some of the themes of tachanun.
It is not said on certain festive days but it is said even when tachanun is not said.
Why is that?
How is the prayer of אל ארך אפים on Mondays and Thursdays different from tachanun?