- You have a Stingmon and an XVmon Digimemory.
- You have an XVmon, and a Stingmon Digimemory.
- You talk to Stingmon in the Train Graveyard with XVmon as your partner, and trigger this event.
With that calendar in mind, it's a good idea to train your Digimon and nab any other potential prerequisites that might gear them towards evolution. This can extend to weight, discipline/happiness, wins, skills, etc.