Just after 12AM UK time on the day before departure, for short haul flights at least, a lot of seats open up for booking relative to the frequency of flights on a particular route. Things that affect this include seasonal routes, route with only 1 flight per day, routes where cancellations have occurred in the days before causing a lot of rebookings.
Outside of the holiday periods, this is what I do for all my short haul flights as it's virtually guaranteed availability on routes such as EDI, DUB, AMS, FRA and MAD (amongst others)