Below is what the OxED says. Note: it was not always 2 days long.
a. The period between two working weeks, typically regarded as a time for leisure or recreation. Also with distinguishing word indicating an activity pursued during this period.
The usual sense.
Since the advent of the five-day working week, the weekend has usually been characterized as extending from Saturday morning or Friday evening until Sunday night. During the late 19th century, a British worker’s leisure period often began on Saturday at noon.
1793 W. B. Stevens Jrnl. 27 Feb. (1965) i. 70 Wrote to Dewe that I would put on my seven league boots next weekend and stretch my course to Appleby.
1844 Age & Argus 13 Apr. 2/3, I am not one of those who think that Almighty God ever designed that human beings should be employed from morn to night—from week end to week end.
1870 Food Jrnl. 1 Mar. 97 ‘Week-end’, that is from Saturday until Monday,—it may be a later day in the week if the money and credit hold out,—is the season of dissipation.
1879 Notes & Queries 5th Ser. 12 428/2 In Staffordshire, if a person leaves home at the end of his week’s work on the Saturday afternoon to spend the evening of Saturday and the following Sunday with friends at a distance, he is said to be spending his week-end at So-and-so.
1905 Spectator 26 Aug. 289/1 What a ‘week-end’ it must have been for the host, whatever it may have been for the guest.
1920 T. S. Eliot Let. Sept. (1988) I. 406, I should have written to Violet to say how much I enjoyed the weekend. We must not let our theatricals drop!
1937 Times 26 Nov. 21/5 The letter began with old Lady Chervil’s unvarying formula:—My dear Mrs. Miniver, Chervil and I shall be delighted if you and your Husband will stay with us from Friday 19th to Monday 22nd November. (She would have gone to the guillotine sooner than use the expression ‘week-end’.
1958 P. Marris Widows & their Families ii. 12 One of the things I miss most is our camping weekends. I used to be as happy as a sandboy.
1996 H. Fielding Bridget Jones’s Diary (1997) 67 Message Jones… Have a good weekend. Pip pip.
2005 J. McKinney California’s National Parks 306 Sometimes 5000 Southlanders a weekend would come to swim, fish, picnic and enjoy the amusement rides.