I've heard mufti before, but probably in British books. I'm sure a fair number of other Canadians would have some familiarity with the word, but AFAIK it isn't particularly commonly used. I don't think I've even heard military people use it (and both my parents, one brother-in-law, and a close friend are either ex-military or still serving). My dad would just say he's in civvies, though two things to consider are that he was enlisted, not an officer, and he's French Canadian; an anglophone officer might use somewhat different terminology.

Mufti day... that's a new one on me, but wyldemusick's explanation of it strikes me as something that would be foreign to a lot of Canadians' experience. I went to six different schools in three Canadian provinces (both Catholic schools and public schools, the two major mainstream kinds of schools in most of Canada) and never wore a school uniform. In the work world I guess the equivalent would be casual Friday, but I don't know whether that applies to the minority of Canadian schools that have uniforms.