Depends on your weather, Nicholas. What sort of temps are you getting?
I would wait a while and make sure the plants are really well hardened off before planting out - then keep fleece handy for any late frosts.
Hi Italophile, its about 16 or 17 degrees here at the mo but dipping to 12 towards end of week
Hi Nightgarden, the celery is zipped under a big cloche at the moment.
Might frost be an issue? If not, you could start hardening them off for planting out in a couple of weeks. Let them out of their cloche, leaving them out later and later till they're comfortable uncovered overnight.
not really got any frosty nights at the mo but ill leave them for a couple of weeks i think to be best