Frost dates are just a guideline to let you know the probability of frost. It's not hard and true. I have known there to be frosts after our frost date.

Yes you can plant frost hardy things earlier. It will take at least a week before stuff comes up if you are planting from seed. You must be a zone 4 or 3 if that's your last frost day.

The test for planting anything is not the date but the temperature of the soil. If the soil feels warm then plant the frost hardy things. We live in Zone 3 and our last frost day is the end of May. That may not hold true this year because we have had cold weather until yesterday. If the weather is normal we plant things like peas and onions and lettuce after the second week of May..

Feel the ground. If it's not warm then hold off planting. Seeds will just sit there in cold ground and may rot if it gets very wet

Ok, many thanks.We've had the coldest March in over 50 years so I think I'll wait a bit. I just have no patience! :)