Winter Grass(Poa Annua) is a weed which germinates around May - June. It must be tackled early, because once established it is very difficult to treat. It's an ugly weed which if left unchecked can completely smother your lawn. If it`s late in the season (August - September) it can still be treated but good results can't be guaranteed.
Winter Grass is a prolific seeder which will distribute thousands of seeds for the next season. The trick is to have it sprayed early.