Rashi on Exodus
רגלים means TIMES. Similar is, (Numbers 22:28) “that thou hast smitten me these three times (רגלים)”.
Sforno on Exodus
תחוג לי, as we find in Psalms 149,2 ישמח ישראל בעושיו, “the people of Israel rejoice in their Maker.” Such joy on the festivals legislated by G’d is to offset the joy displayed on the occasion of the making of the golden calf, when the Torah reported “when he saw the calf and the people dancing, etc.” (Exodus 32,19).