Fall is the best time to care for your lawn. You will get a better kill on perennial weeds now than in the spring. The weed in the fall will be taking more nutrients down to the root for the winter and if you spray a weed killer now, they will take that down to the roots also. Use a chemical like Weed-B-Gon other any 2,4-D based herbicide. You can spray until a hard freeze (28 degrees).
Fall is also the best time to put on a fall-based fertilizer. This will help the lawn regain strength, especially after the long hot summer we had for better spring growth. You can apply anytime until the first of November. Mow the lawn before applying fertilizer leaving the grass clippings to insulate the lawn for the winter. Make sure the grass blades are dry and water afterward.