I grew up in the south where it gets hot a little sooner, so my mom's rule was Easter instead of Memorial Day for white as well as for anything made of linen. And Labor Day was the start of "fall clothes" for us.
Fashion Houses produce "winter whites" year round. They're heavier fabrics and, often, just a hue off- maybe more ecru.
However, some places differ because of climate. For instance, you see movie stars wearing white year round- but remember it's usually warm and sunny in LA.