Once there's no danger of frosts and the weather is generally warmer and drier, you can do it. As they've been cosseted indoor pigs over winter, it would be too much of a shock to their systems to go out if it's cold and damp. Easter is probably fine.
We usually wait till Easter time (though this year it's early). Mine have a Pighouse, so they're under cover anyway, but at the moment they're in at night (in their hated indoor cage) I know last year they were definately out by Easter because DH was looking after them (and he didn't need to put them out)
It's the damp weather that I'm most worried about though. I can cold-proof our hogs with extra bedding and a small heater on tickover setting. But I like my hogs to be warm enough that they don't want to just sit in their hay. They need to potter about.