If you want to have a great lawn next year, the time to spray for weeds is the fall. Weeds start to grow in the fall, then go dormant over the winter, and fully emerge the next spring/summer. If you know you have an issue with specific weeds every year, such as dandelions, chickweed, or henbit, you can spray for them when they are growing back with the warm days and cool nights that fall brings. If you have any spots where the weeds grow in clusters, or areas of your yard that are prone to weeds, this is where you should focus your spraying. Make sure you use the proper herbicide, and determine the right time as well, if you are spraying pre-emergence or post-emergence.