"This would happen every winter."
Translation:Questo succedeva ogni inverno.
Sorry. Questo is refering to an unspecified "this", and, when gender is unknown, one must assume the masculine. I'm not even sure that "questo" can be used as an indefinite pronoun anyway, but that's an entirely different kettle of fish.
So, in conclusion, you must use "questo" precisely becuase the gender is unknown.