When to prune Lilac trees

I have read a number of reports on how to prune Lilac trees, however I would like to know the best time to prune. I am refering to South Western BC, Vancouver area.
Bryan

 

after they are finished flowering

 

Thanks for info. Bryan

 

After flowering, remove dead branches, and flower heads. Only remove up to a third of the plant, per season. Wood ashes &/or Lime, will greatly improve them, after this is done.