In the Northern Hemisphere, the best time to be vaccinated is between September and December, ready for the winter months. The peak period for flu transmission is from late December to the end of March.
Ideally, you want to be able to get vaccinated and allow for up to 14 days after vaccination to develop antibodies before coming into contact with the flu virus. This is much easier to do when the number of flu cases are low. As flu is highly contagious the more community transmission, the harder it is to avoid infection.
If you wait until there is a flu epidemic in progress, it may not otherwise be possible for you to get good protection quickly enough and in some rare circumstances, flu vaccines can be in short supply so we advise early vaccination for peace of mind and maximum protection.