A junction is (1) a place where two things join or meet, or (2) the act of joining or meeting. Juncture has traditionally borne these senses as well, but in modern usage it more often refers to a point in time, especially one made critical by a confluence of circumstances.
Because critical is contained in one meaning of juncture, the phrase critical juncture might be considered a redundancy, but it seems that the critical part of the word’s sense is not widely observed. In fact, in web-published newswriting from this year, juncture is paired with critical more often than not, suggesting that, to many English speakers, juncture simply means point in time.
In these sentences, juncture is simply a variant of junction:
[S]she was surprised to learn that “such a quiet town” of about 2,500 residents at the juncture of routes 22 and 66 holds such a prominent place in local history. [Tribune-Review]
[I]t left my right eye vulnerable if burning powder sprayed from the juncture between the bolt and chamber, which seldom, if ever, happens. [Portland Press Herald]
These writers use juncture to refer to a critical point in time:
[W]hile she’s “fully aware” of the difficult juncture they are in, they have few options but to persist with fiscal adjustment. [Irish Times]
If confirmed, Dunford will preside over the war in Afghanistan at a challenging juncture. [Washington Post]
It is at this juncture that we have to be very careful with any suggestion of decriminalisation. [Telegraph]
And in each of these sentences, juncture refers to a point in time that is not especially critical:
What made him go there at this juncture in his life, 31/2 decades after the Band packed it in? [Los Angeles Times]
What I’d recommend to anyone seeking out a new podcast at this juncture is a look through the comments of the previous post and see which catch their eye. [Make Use Of]
Rivers, a 6-foot-3 guard headed to Duke, put on the best show, draining seven straight 3-pointers at one juncture and making several great passes. [Chicago Tribune]