The morning will be best, particularly if you can get some resistance training in as well.
A workout elevates your metabolic rate for several hours after the actual session so you will feel the benefits throughout the day.
Even better, if you can get an early morning in and then a quick turbo charge at lunchtime - a 20 minute strength circuit should fit nicely in before you have lunch and will make sure that that calorie burn continues throughout the afternoon too!
Rod
Crossfire Elite Personal training
http://www.cfire.co.uk