Fredagsmys literally means 'Friday cosiness'; think TGIF, but fewer cocktails and more blankets and snacks.
Mys is related to the adjective mysig (cosy) and the verb att mysa, which can mean 'to cuddle/snuggle' but usually just means 'to have a pleasant, relaxing, chilled out time'. You'll also occasionally hear things (a film or recipe) described as fredagsmysig or hear people use the verb fredagsmysa (to have a cosy Friday night in).
There are plenty of different varieties of mys: höstmys, julmys, tacomys, kvällsmys, strandmys and so on.
But fredagsmys is one of the most commonly used forms. It was most likely coined in the 1990s to describe an evening spent relaxing at the end of the five-day working week, first appearing in Swedish media in 1994 and entering the Swedish Academy's dictionary 12 years later. There are equivalents in Norway (fredagskos) and Denmark (fredagshygge).
Order and routine are a big part of Swedish culture: pea soup is traditionally eaten on Thursdays, and sweets traditionally eaten on Saturdays (known as lördagsgodis or 'Saturday sweets' – this one actually has a scientific background, as studies showed that eating one large portion of sugar each week was less damaging to teeth than a small amount eaten more regularly). So it's only natural that their weekly downtime be given a designated day too; and it highlights the value given to a healthy work-life balance.
Fredagsmys usually means getting cosy on the sofa with some easily-prepared food (often tacos or pizza, fizzy drinks, crisps and sweets) and watching TV. There's usually another person involved: you can mysa with a partner, family, friends, or pets.
There's even a fredagsmys anthem, a song created for an advert for crisp company OLW, which stresses that this is an activity for people of all ages and professions. Perhaps expats will relate to the English-speaker who says 'since I moved here, all I do is mys'.
Nu är det slut på veckan, det är dags för fredagsmys
Now it's the end of the week, it's time for a cosy Friday night
Jag älskar att fredagsmysa
I love having a cosy Friday night in