Facebook Users' activity heatmap reveals when users are most active on your Page. The heatmap shows the busiest day of the week, as well as the busiest time of the day. The darker the tile on the heatmap, the busier the time/day is, in terms of user activity.
Hovering over the heatmap you'll see the number of users' interactions for a given time of the day.
The activity of users on your Page is based on the time and date of their comments, messages and posts.
Note: The displayed time is based on the timezone selected for the profile. This can be found in: Profile settings > Reports > General > Timezone.