It depends on what plants you are planning to plant. In our general area, the rule of thumb is after Mother's Day for warm weather plants. Given our erratic spring weather, and forecast for a cooler May, you need to look at the extended forecast for your specific location. Also, since you are using grow bags, the plants will be exposed to air temperature. To be safe, the soil temp should be above 60F and the night-time air temp should not go below 50F for most plants.

Even if there is no frost, warm weather plants can suffer from cold soil and chilly nights, impacting root growth and possibly leading to blossom drop later on. Two ways to warm soil faster: black plastic covering the bag and/or placing the plant bags near a south-facing wall.

Depending on the plants, I would wait until after May 15 to plant, but you can put soil into the grow bags, cover them with black plastic and let the soil warm up.

You could also do an internet search for your specific plants. "extension, edu, plant name (Latin name if known), recommended soil and air temperature (or ideal growing conditions)" By using extension and edu you should get vetted info from a university site.

Good luck
Len