Rashi says, “In the garments in which one cooked for the master one does not pour the master’s wine”. The sages comment, “Hence one should remove weekday garments and wear special garments for Shabbat” (Shab. 114; Maharsha’s commentary).
In these days of relaxed sartorial standards this is a timely reminder that special occasions require special clothing. Coming to synagogue on Shabbat or festivals, for example, should not be in weekday and workday attire, or in the clothes one wears for sport or to go to the beach.
Rabbis have seen it all: women in skimpy clothes with no thought of modesty, men in shorts and sandals (on Kol Nidrei night I have actually seen a man in shule like that).
As a “kingdom of priests and a holy people” (Ex. 19:6) the rule in the sidra should be our criterion.