What a good question! Thanks, Mal! *waves enthusiastically*
When it comes to veg and seedlings, all the books and articles will tell you to "plant them out once all chance of frost is over" but who knows when that will be?
Certainly, this year, it's been a blooming cold start to spring: this time last year, I'd been back in shorts for a week. But this time the year before, one of my friends reminded me that they were milking cows in the snow!
Ah, the joys of living in jolly old England, where the weather is so unpredictable. I spent some time in the Czech Republic a long, long time ago, and I was staggered that they didn't bother with weather forecasts: they knew that whatever weather they had now, it was going to be the same for weeks on end. How dull!
So what do we do about the late frost problem?
Two things, really: keen gardeners will always keen an eye on the weather forecast, and if it looks like being frosty overnight, they rush out with lengths of horticultural fleece, and wrap up or cover any sensitive plants, or delicate seedlings.
The other way to deal with it is to do a rigorous "hardening off" process: that's where you gradually accustom your seedlings to the outdoor weather.
Here's the routine: for the first week or two, starting about now, put your seedlings outside in the sun during the day, but bring them in every evening.
On the third week, you can stop bringing them inside at night, but you must cover them up with horticultural fleece or something similar, over night.
On the fourth week, they should be fully hardened off and can then be left outside all the time, until they are big enough to be planted out.
Unless, of course, it turns really cold again, or you hear that frost is forecast, in which case you have to go back to covering them up at night.
Now, like all gardening advice, this has to be taken with a pinch of common sense: if you live in an area which is very windy, or which is prone to frost, then you might need to extend the hardening-off period for a few weeks more. But if the days are balmy, and the nights are merely cool, then you might be able to shorten it.
How will you know? Only by trial and error, I'm afraid. If in doubt, follow the regime slavishly. If feeling bold, do what you think best.
If you are undecided, then take a half-way position: maybe leave half of your seedlings uncovered, and see if they survive!