Flags may be flown on every day of the year. Government and local authority buildings in England, Scotland and Wales are encouraged to fly national flags every day of the year (there are certain additional regulations in Northern Ireland).
Flags are normally flown from sunrise to sunset but they may also be flown at night, when they should be illuminated.
No permission is needed to fly the national flags and they are excluded from most planning and advertising regulations (but flagpoles may not be).
This document explains the procedures in more detail and can be downloaded here: