You can have your pond cleaned out at any time of the year, really the only difference is we get cold and wet in the winter compared to the summer! The one thing to be mindful of is if your pond is home to wildlife. Frogs and Toads will spawn between January and March and Newts will spawn between March and June. So, with this if you have a population of these it may be worth getting your pond cleaned out before these months or a couple of months later to avoid disrupting them. Although being said it is easy to move spawn into holding tubs during cleaning so if you want the pond cleaned during these times it can still be done.