AARP suggests that Wednesday from midnight to 1a.m. in the time zone of the airline’s “home base” is the best time to purchase a ticket directly from the airline.
The reason being the airline software will cancel all unbooked reservations – that includes discounted fare hence you will likely to see some cheaper fare. Not sure if it works in countries other than US but I am sure it is related to the software they use.
Tip For Best Purchase Time of Airline Tickets – [AARP through Sound Money Tips]