Is tirsdag supposed to be pronounced tishdawg?
No. It is supposed to be pronounced: "tirshdag". The "r" sound won't be completely muted, you ought to pronounce both "r" and "rs" sounds.
Isn't it a mistake? Hverdag means "everyday".
hver dag = every day; hverdag = a working day
Wow! Another mind blown. Takk !
In a few situations I would say you could safely translate hverdag to "everyday". Example: "Han har en vanskelig hverdag" would be understood as similar to "His everyday life is hard/difficult".
Are you sure you write it without space between hver and dag?
Hverdag is without space between the two words unless you want to say every (single) day. See comment by Griffindd where the usual translations are given.
Do Norwegians actually use "hverdag" as "a working day"?
I have always used "en abreidsdag" (as a working day) and "hverdagen" (as everyday/daily life). I guess it's not a mistake, but I'm wondering what is more common?
Likely stems from the old idea that every day is a working day