As greene says, we really need to know where you are to advise you.
I'm in Florida and can't grow tomatoes here in the summer - the nights are too warm and the plants won't set fruit. So I'd say once you have nights that fall below about 70deg. F you can plant out transplants. Raise them indoors for a month's gain on the process if you have a sunny window. I plant my tomatoes in September or October, protect them a bit if we get a few cold nights, and harvest until just about now. My current plants stopped setting fruit about a month ago.
Cucumbers? I've entirely given up trying with any cukes, melons or squash. The fungus and bugs make it just too much of a battle to get any harvest.
