if it is pot grown then any time you want to. If you are lifting from one area of a garden to another then you need to take as much soil as you can round the roots. then water well. especially if this hot dry spell continues.
if its the corm in a recently bought package from the garden centre then I'd put it into a pot until it is growing actively then transplant it into your garden. If it is the plant I think it is then it can become invasive.

Thank you.

they do become invasive , but me and my neighbours love em, i had growing all over the place . i em had in lots of pots, which i needed for other plants for my patio, i emptied the pots near my compost heap, this year i got loads of pretty flower aroud there as well. one of my best buys i think

What is the latest date you can plant them in.