I‘ve only been to one, so I'm sure @MinuteWalt and others will correct me if I'm wrong, but unfortunately they do usually occur in concentrated city centres.
Anomalies occur over longer periods of time (weeks maybe?) and take place simultaneously all over the world. People attend and pour time and resources into battling for team control in special localized battle zones. (Judging from the PoGo trailers with people battling MewTwo in Times Square, this seems the most similar, although the timing coordination seems more difficult.)
Mission Days are when organizers would create missions, or little scavenger hunts, where you had to go down to a specific location and hit all the portals along a given path. If you were successful in completing the majority of the missions, you'd get special badges as awards.