You can create a pond at any time of year but it is important to bear in mind the wildlife that you might be about to disturb when you make a pond, either when the ground is dug up or the site is driven over.
Birds could be nesting in trees and scrub or on the ground so avoid working in the nesting season or check thoroughly that there are no nests before starting work.
The main consideration is how wet or dry the soil is and therefore how much hard work it will be to dig. If you are making a large pond or using a digger, then bear in mind if the machinery can get on site without getting bogged down.
