The year was 1901. Read more about naming customs in this article written by Ingela Martenius.
http://web.comhem.se/~u31263678/genealogy/Names.pdf But in 1901 an ordinance was issued - which in fact was only an addendum to the ordinances regarding church records - that stated the proper procedure for acquiring a family name: all “unique” names had to be approved by the authorities, who checked that you did not use someone else's name. If you did not want to use a “unique” name but instead e.g. you patronymic, you could do so without having to make any special application for it. You were however not forced to take a family name,
but society had in general changed so much that this was now becoming the
norm.
Also look at SweGGate.
http://rootsweb.ancestry.com/~swewgw/Fact/Name/facname1.htm In 1901 the first law about surnames was passed. In effect it outlawed the earlier custom using patronymics (see below).
(1) The norm became to use the father's surname for all children regardless of the child's gender.
(2) A child born in wedlock receives the father's surname.
(3) A child born out of wedlock receives the mother's surname. The father, or a man (later) married to the mother, can allow the child to use his surname.
(4) Adopted children receive the surname of the adopting person. By court permission an adopted child may use his/her surname at birth alone or in conjunction with the surname of the adopting parent.
The legislation regarding names has been revised / extended many times after 1901, notably the major, coordinated revision in 1963.
Also from SweGGate (on the same page):
Effect of the 1901 name law:
The literal meaning of a patronymic surname is lost. Karin, daughter of Karl Svensson, would be named Karin Karlsdotter if born before 1901 but Karin Svensson if born after 1901. This is important for genealogists. It is often useful to create a patronymic surname from the father's first name or deduct the father's first name from a known patronymic surname of a child born before the end of the 19th century but almost always leads you wrong regarding children born after 1901. In the latter case you should look for a father with the same surname as the child.
Judy