The season goes from April to November. In that time air from spring 10-20°C, in summer up to 30°C and autumn 10-16°C. Water spring 8-13°C, summer 18-23°C, autumn 8-14°C. You always need a wetsuit. You can go out off-season but it will be very cold, a thick wetsuit or better drysuit, mask, shoes and gloves recommended.
Take E55 south from Kobenhavn. Exit to Fakse and take 209 south to Vingbyholt and follow signs to the peninsula Feddet. Park anywhere on the east coast to access the beach.