Answer: You can still plant container grown plants now, but the sooner the better as the summer is fast approaching. You may want to work on the lawn area and weeding this summer to see which existing plants are worth saving. (Many neglected plants can be brought back to health with fairly minimal care.)
Your first steps might be to run some basic soil tests and fertilize and/or lime the lawn according to the test results.
Mow often enough that you do not remove more than one third the grass height when you mow; set your blade high at three inches. These three steps will go a long way toward restoring the lawn. If it still needs work, surprisingly late summer is the time of year for extensive lawn work.
Weeding can be done by a combination of hand pulling (roots and all) or using an herbicide containing glyphosate (read and carefully follow all of the label directions.) You can also smother weeds under a layer of newspaper topped with organic mulch.
If you want to do flowers, remove existing grass/weeds, loosen the soil down about ten inches, work in several inches of organic matter such as compost or old rotted leaves, then level and rake smooth. It is a bit late in the season so you would probably want to use transplants. After planting, mulch the bed with organic mulch. Apply it in a layer two to three inches deep.
Your local Cornell county extension should be able to help you with the tests and interpreting the results. They may also have additional suggestions on bringing back your landscape. Good luck with your project!
Q&A Library Searching Tips