In northern Utah the last day of frost is generally around May 15. After that it is consider safe to plant tomatoes, squash, melons and other plants that are not cold-tolerant.
However, this has not been a typical year. Last winter was mild and so far spring has been unusually warm. Right now we are seeing record warmth. Will the warm weather last?
It is a bit of a gamble but many gardeners are planting now. Get some small plants into the ground now for early crops. If needed, you can cover small plants with bottles to protect them on frosty nights.
But the forecast right now is for continued mild weather and so we may have already seen our last frost of the season. For me it is definitely worth the risk. Gamble a few dollars worth of plants now and you will probably be eating fresh produce weeks earlier than normal.