The mantra ‘deadlines are sacred’ should already be part of your day as a freelancer. No editor wants to work with a writer who can’t turn in material on time and no other client will give you repeat business, either. But that’s their deadline.
My deadlines tend to run a bit ahead of everyone else’s if I can pull it off. Telling myself that an article is due a week before it actually must be in gives me a little padding and can impress an editor if I’m consistently early. But it pays off in other ways.
Are you a fiction writer? Say that you’ve found an anthology you want to get a short story in. You get it in towards the end of the submission period and get an automatic response that the editor’s filled all the spots, but they’ll keep you in mind for next time. Platitudes aside, getting that story in earlier would have been helpful.
What about queries? Say you’ve got a perfect idea for a Christmas story. But most magazines decide on content months in advance. If you wait until November to query, you’re just out of luck.
The list goes on and on. Do it early. Get it done. You’ll be thankful in the long run.