Senjuushi: Kijuushi-tachi no Happy Birthday!
Japanese title: 千銃士 貴銃士たちのハッピーバースデイ!
Alternative titles: Senjuushi OVA
Type: OVA
Source: Game
Episodes: 1
Status: Finished Airing
Aired: Mar 6, 2019
Duration: 24 min
Rating: PG-13 - Teens 13 or older
Studios: TMS Entertainment
Genres: Action
Themes: Anthropomorphic, Military

The story will be about the characters celebrating their master's birthday, which happens after the sixth episode. 

(Source: MAL News)