the middle of March, when the maples bloom.
Our maples bloom ih the middle of May.
I try to start protein feeding 3 weeks before willow bloom starts. It mean that first big group of bees emerge when nature gives new pollen.
The earliest time to start is when half of snow is melted. If it too cold or snow on ground, bees cannot carry water outside, larvae will die. Often bees get a bad chalkbrood.
When mapple blooms here, I begin to enlarge my hives. 2-box hive gets it's third box and 1-box hive gets it's second box. Fist size colonies get so much emerging bees that they can occupy one box and make it full of brood.