You don't say where you are or what sort of grass you have, but this sounds like red thread disease, which is generally best controlled with proper maintenance. Keep lawn well fertilized, don't water in the afternoon or evening (to avoid excessive or prolonged exposure moisture) and dethatch, if you seem to have a thick layer of thatch. By summer time, when weather warms and isn't so damp, the disease will likely disappear.
There are fungicides that garden centers carry for this, but try these suggestions first. You're better off doing this without the fungicides, unless it's so severe that you're about to lose large swaths of lawn.