Never book flight on Saturdays. Usually on Saturdays prices are highest. Although only then you have time to organize trips. Airlines follow the weekly price system and may seem confusing and frustrating for passengers who do not know how it works.
Although now mostly everyone book on the Internet, the system is still the same. At the time relevant for pricing they watch available flights on Monday to do better offer during the week. If there are many unsold tickets at the beginning of the week, the price goes down.