Spotify uses 28 rolling days (4 weeks of 7 days) to measure a month. It makes sense as the number of days in a month varies from 28 to 31, and people’s behavior depends more on the days of the week than on dates within a single month. This way, there is always an equal number of each day within the 28 rolling day ‘monthly listeners’ stats.
We update the global monthly listeners stat each day. Once a week, the endpoint also returns Spotify's "Where people listen" stats, showing the top 50 cities for monthly listeners.
The API response may not return a value for all dates, as we may miss a day or two. These data points may still show on our UI because we are interpolating the missing data between two points.