Niantic has put together the below post to explain exactly what Pokémon GO Community Days are:
What are Community Days?
Community Days are once-a-month events, each featuring a specific Pokémon that will spawn at a higher rate than other Pokémon over during a specific time frame per timezone. These featured Pokémon are often rare or uncommon spawns, making Community Days a great opportunity to encounter and capture as many as you can! Featured Pokémon evolved to their final evolutions for the hour following these events will learn an exclusive Charged Attack, unavailable outside Community Day events.
Aside from increased spawns, every Community Day brings gameplay bonuses for your location’s duration of the event. Some examples of previous Community bonuses are:
- Increased Stardust rewards
- Increased Catch XP rewards
- Increased Lure Module effectiveness times
- Decreased Egg hatch distance
For the best Community Day experience, please ensure that you have a strong internet connection, set your GPS settings to the highest accuracy for your device, and are using your device’s automatic timezone. Because Community Day is a location-based event, you may experience issues if your clock or GPS are inaccurate or fluctuate.
Community Day events are announced in advance through our site, alongside information on the featured Pokémon, exclusive move, date, and bonuses! The timing may differ for your location from month to month, so we highly encourage interested Trainers to check out the site regularly for information about the next upcoming Community Day.
Source: Official Pokémon GO support page