I assume the OP is referring to the Grand Masters Training Mythic event, not the Ground War Assault Battle.
According to SWGOH EVENTS, it runs at an impossible to predict frequency. There were two runs in December because of the holiday event schedule, but it was almost 5 months between those and the previous events:
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According to SWGOH EVENTS, it runs at an impossible to predict frequency. There were two runs in December because of the holiday event schedule, but it was almost 5 months between those and the previous events:
Grand Masters Training Event
This is swgoh. That's how you handle most things in the game 😂
RNG plays no part in it. Event scheduling does.