None.
If I fertilize I just need to cut it more often, if I apply weed killer it would kill the clover, crocus, scilla, and violets (and, dare I say it, dandelions) I try to encourage in the lawn. We keep bees and maintaining as wide a diversity as possible of abundant flowers, especially in early spring seems wise. Plus it's really pretty.
I do use a crabgrass pre-emergent once every few years to keep the stuff at bay.
Home lawn fertilizer is a major source of nitrogen pollution that screws up our streams and ponds, and as far as I can tell is completely unnecessary. Clover stays much greener through the summer and puts nitrogen into the soil to support the grasses there. Plus the bees love it.