Weeds in lawn
If this greens up each year, it is a perennial grassy weed, likely bermuda/wire grass, which creeps, or nimblewill, which forms slowly expanding circular patches.
Here is our lawn weeds gallery page where you can see them and compare: http://www.extension.umd.edu/hgic/lawns/lawn-weed-identification#grassysummer
Click on the photos for more information.
If you have a lot of weeds in your lawn, it may be time for a lawn renovation next late summer into fall.
Here is our lawn renovation page which includes links to our publication which covers all the particulars on how to go about this: http://www.extension.umd.edu/hgic/lawns/lawn-renovation
What do you recommend to eliminate small patches ?
Control options are covered under the weed gallery pages given above.
For instance, here is the wiregrass page: http://www.extension.umd.edu/hgic/bermudagrass