Not until mid May. Tomatoes don't like cold, so the soil needs to be warm. May frosts are rare, but can occur. Just keep on eye on the weather forecasts.

The tradition is Memorial Day (that was original Memorial Day, May 30). Our official last frost is May 15, but we pretty much never have frost that late. But we can have very unsettled cool weather in May, and sometimes even into June. Tomato plants hate cold weather, so you plant them in very late May, but if it's chilly and rainy, you can wait longer.
Lots of times there is beautiful weather in mid-May, and the garden centers are full of people buying all their tender annuals. They plant them and then and it's miserable and cold.

I put mine in around the first week of May. I start them from seeds so I try to harden them off before planting.

York is South of Reeser's Summit.

If you are North of Reesers Summit, wait a bit longer than Julie. You will not be sorry.

I put in first week of May and if there is a danger of frost just cover them with a plastic sheet for the night. Of course you can wait till mid May to be sure,but I like to get it going.

Soil temp. is more important. If they're put in soil that is not warm enough they'll just sit and sulk. This is why tomato plants can last all the way til frost, sometimes mid-November here.

We've always used Mother's Day as the yardstick, which is pretty much mid-May.