Official xkcd meetups happen every Saturday afternoon at 4:00 p.m.(*) at that day’s normal geohash coordinates. It is considered a Saturday meetup if you are there at 4:00 p.m. or if you meet people, but you are most likely to encounter other readers of xkcd if you go at the designated time. Bring games!
You're encouraged to geohash on any day of the week that the coordinates are accessible to you, but if you attempt one meetup all week, make a little extra effort to have it be on Saturday. All meetups start at 4:00 p.m. or as designated on the graticule page.
There's even an achievement for making it to 100 Saturday meetups.
*In some areas, 4:00pm is too close to sunset during the winter, so earlier meetups are often more appropriate. See individual graticule pages for local conventions.