If you can start celery seeds with heat in February that is ideal, otherwise unheated in March will be fine. You should sow seeds into large modules or small pots and planted out at final spacing in May. Harden off and cover with fleece to protect from cold after planting. Celery and celeriac are evolved from a marsh plant and are therefore very thirsty, so be sure to water them in dry weather. Celeriac needs a long season to produce decent-sized roots.
Celery originated in Eurasia and was originally used medicinally, records of its use date back to over 3000 years ago. It was most likely grown for its seeds before being adapted as a vegetable.