Niantic have announced the details for the July 2020 Pokémon GO Community Day which takes place on July 19th, 2020 at 11:00 a.m. to 5:00 p.m. local time. The featured Pokémon for this day is Gastly. Gastly will appear in the wild more frequently for six hours that day. The 1/4 Egg Hatch Distance and 3-hour incense bonuses will also be available that day. If you evolve a Haunter during the event or up to two hours afterwards, you will to get a Gengar that knows Shadow Punch.

Source: https://pokemongolive.com/events/community-day/

Catch Gastly and Get a Gengar That Knows Shadow Punch during Pokémon GO’s July Community Day

Stay on the lookout for the Ghost-type Gastly, and take advantage of awesome bonuses.

The upcoming Pokémon GO Community Day is your big chance to get Gastly. On Sunday, July 19, from 11:00 a.m. to 5:00 p.m. in your local time zone, this Gas Pokémon will appear more frequently in the wild, and there'll be a better chance those you encounter will be Shiny. Plus, if you evolve a Haunter (the evolved form of Gastly) into Gengar during the event—or up to two hours afterward—that Gengar will know Shadow Punch, an attack it can't normally learn.

Additional bonuses will be in effect, too, such as Incense activated during the event lasting for three hours.

Source: https://www.pokemon.com/us/pokemon-news/catch-gastly-and-get-a-gengar-that-knows-shadow-punch-during-pokemon-gos-july-community-day/