Ballista Watcher // Ballista Wielder

• WOTC Official Reminder Textofficial rules
  Daybound — If a player casts no spells during their own turn, it becomes night next turn.
• WOTC Official Reminder Textofficial rules
  Nightbound — If a player casts at least two spells during their own turn, it becomes day next turn.

• WWizards of the Coast2021-11-19
  Official Ruling
  The activated abilities of Ballista Watcher and Ballista Wielder are completely independent of one another. For example, if you activate Ballista Watcher's ability targeting a creature and then it becomes night, the targeted creature can still block this turn. Conversely, if you resolve Ballista Wielder's ability targeting a creature, transforming it into Ballista Watcher won't allow the creature dealt damage this way to block.

Rulings via Scryfall