The seasons are determined by
The seasons are determined by the relative angle of the Earth to the Sun. This does not necessarily mean that after the Vernal Equinox it will suddenly be warm and flowerly and spring-like. It just means that the days will get longer in the Northern Hemisphere, and shorter in the Southern Hemisphere. Historically, since the last ice age, anyway, days start getting warmer after the Vernal Equinox, except in extreme locations, like on top of mountains, or in deserts, or near oceans.