Japanese stilt weed in the lawn
When a lawn is more than 50% weeds, it should be renovated, i.e. killed and reseeded. The best time to reseed is early fall, beginning in late August. You'll want to kill the lawn with a herbicide before that. Also, get a soil test done now so that you can add any lime or fertilizer it needs before you plant the grass seed.
Here is a good guide to a successful lawn renovation: http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/lawn_pubs/HG102...