Depends where you are I am in Scotland and never put mine out until 2nd or 3rd week inJune when frosts should be no more.

But who knows this year.

Bitter cold and sleet showers again.

Even my winter pansies are growing behind glass in greenhouse.

When all risk of frost is past - and if you know when that will be, you're a wonder! But the standard advice is end of May, assuming your basket is planted with summer bedding.

ive got 2 out now. and we have had slight frosts , but the plants haver not been damaged, but otherwise in the next 2 - 3 weeks will ok

They say the end of May for a good reason - we have, in fact, had overnight frost in the south on 4th June before - but the plants would survive just one slight frost, so end of May it is. Here in London, I put them out mid May, unless its particularly cold.