They use the calendar year. January to December.
FD's Fiscal year ends 31 May.

Just to throw another spanner in there. Can't have too many, can we?
As I originally stated. Most people do not use those date for the seasons in the UK. Those are astronomical dates which 99% of the UK population do not use.
But we're space men in SPAAAAAACE. Wouldn't we use astronomical terms?