Check your yard for any standing water that may have accumulated in containers and other places after it rains or after watering in your yard. Mosquitoes are attracted to stagnant water and will lay their eggs on the surface. Mosquito larvae will hatch out after several days and will develop into adult mosquitoes. Be sure to dump/drain/empty the water to kill the mosquitoes. No water = No mosquitoes!
Follow this link to see a short video of mosquito larvae in action; you'll see why they're also called "wigglers":Mosquito larvae