(Exodus 23:14) commands: "Three festivals shall you celebrate for Me in the year." The Mekhilta asks why this verse is needed when (Exodus 23:17) already says "Three times shall every male among you appear before the Lord."
From verse 17 alone, one might think the three appearances could happen at any time or place the person chooses. The obligation is to appear before God three times — but perhaps the specific occasion and location are flexible.
(Deuteronomy 16:16) corrects this: "on the festival of Matzoth, and the festival of Shavuot (the Festival of Weeks)h, and the festival of Succoth." The three appearances are tied to three specific festivals. You cannot substitute a different time or occasion.
Verse 17 adds another dimension. The phrase "shall be seen" — in Hebrew, yod-resh-alef-heh — is ambiguous. It can be read as "shall be seen" (passive: you are seen by God) or as "shall see" (active: you see God). The dual reading suggests that the festival appearance involves both seeing and being seen — a mutual encounter between Israel and the divine presence.
The Mekhilta thus establishes that the three annual pilgrimage festivals are fixed, named, and non-negotiable. They are not generic appointments but specific encounters, each with its own character and timing, rooted in the agricultural and historical calendar of the Jewish year.