Payless

The majority of travelers hold jobs Mon – Fri working hours, and 2 weeks of paid vacation. If you have more than two weeks, you most likely have a job in a big company and you have been working for a while there. People are happy for every day they can squeeze and make their vacation longer. That is why travelers usually choose to leave on Friday evening and come back on Sunday evening. Airline ticket prices are based on availability: less availability higher prices. Like everything else based on demand and supply.
Also airlines always set the fares with weekend surcharges: travel Friday, Saturday, and Sunday is minimum $ 25 surcharge per direction. British Airways consider weekend travel Thursday through Sunday and has surcharge of $ 30 per direction. Also you have to add the fact that on the weekends is busier and probably the flights will be booked. Of course this is not always true. Sometimes you might get better fare traveling on weekends. If you have flexibility with the travel dates chances are you can get the lowest price possible for the period of time you need to fly.