Kick off your search with a price comparison website. Sites like Skyscanner allow you to be as specific or flexible as you like when you’re searching - you don't even have to give a destination; simply browse by your chosen date and take your pick from the best flight deals. If you do have somewhere in mind, you can pinpoint the best time to travel to your chosen destination.
It can help to book in advance, especially for holidays during popular summer months and school holidays. But, if you miss your chance to get in early, you could still grab a last minute deal. Sales for weekend flights usually start on a Monday, but as more competitors list their deals, prices can drop further so Tuesday evening can be a good time to snap up a bargain.