seed or fertilize first?
When you seed, you can fertilize with a starter fertilizer. Your existing lawn should be fertilized, too, but not with starter fertilizer.
Late August-early September is the best time to seed a lawn, because it gives the new grass seedlings plenty of time to get established before freezing temperatures hit. Mid-October is the latest time we recommend seeding, though the warm temperatures this fall, and prediction for the winter, may make a seeding now successful.
Please read through this publication for seeding success: https://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG102%20L...
There is also a weeds section on our homepage that you should find helpful.