Based on the title text from the comic that established geohashing, the "official" meetup day was interpreted as being Saturday; that is, the day one would have the best chance of meeting others -- see also Mouseover Day. Additionally it was decided through convention that a good meeting time would be 16:00 local time (4:00 P.M.)¹
However, neither of these are hard rules, and they were formulated at a very different early stage in the sport's history. Nowadays and for quite awhile actually, any date or time can be good (or bad, depending on how many other hashers are near you) for meeting up, especially if prearranged. Note that this only applies to that day’s normal local geohash or globalhash coordinates, if you try to go to an alternate location without telling anyone else, it's highly unlikely you'd meet up with a hasher there (obviously).
¹Or earlier if that would be too close to sunset during the winter, or other quirks of temporal tradition; see your local graticule page for consensus there.
