Either could work. If the gathering is referred to as Taco Tuesday and it’s a known or recurring event, you can say you’re meeting friends “for” Taco Tuesday which suggests you’re actually going to the event and it can be at different locations . If every Tuesday you meet for tacos at different places (or the same place) but it’s still the same event, you could say “for”. E.g. We’re meeting for Taco Tuesday...this time, it’ll be at XYZ restaurant.
You could also use “at” which does not necessarily mean you’re planning to go to the event, it’s just the location you’re meeting at but you could be then going someplace else, or you could be staying.
In the second example you gave , if “Tacos on Tuesday” is the name of the event, it’s fine, other than you use “a” not “at”.