While the grass is dormant during the summer months, can you use a grass and weed killer on your lawn ...?

The 2 prior answers are good with good suggestions. If you are going to spray the weed, It must be done during the cool time of the day. Plants are similar to Human Beings. When the temps get too hot, typically 85 degrees and above, they shut themselves down and will not accept nutrients from the blades and stems. They seek shelter from the heat, much like we do. If you do treat the weed with a herbicide in extreme conditions, you're liable to do more damage than good.

If what you have is creeping charlie, the ONLY time an herbicide is effective is when it's actively growing. Just go to a local garden center NOT HOME DEPOT, and tell them your problem. They will, Gladly, tell you what to use, how to use it, and when. Be prepared though. It does take 2 applications.

Call your county cooperative extension office, ask for the horticulturist or agriculture educator on staff, they can id the weed and help you get rid of it.

the weed that you have is a noxious weed that not killed by traditional pesticides. talk to a local nursery for more advice. i have had the best success by picking it out by hand before it flowers. i eliminate any areas where weeds are likely to grow by landscaping.