When was the Canon first published for public use?
The canon of scripture actually developed over centuries through a process of consensus. There was not a single moment or decision that “ratified” the canon. Rather, churches started reading texts as a part of their worship services. Initially, there was quite a bit of diversity. But as time went on, for various reasons, a consensus developed over what constituted scripture.
The early evidence we have for the canon doesn’t amount to much. It includes sources like the Muratorian Canon, a source of unknown date, which lists 22 (rather than 27) books of the New Testament, and also includes 3 additional works that aren’t in our current NT. Another source, the Synod of Carthage in 393 C.E., lists all 27 books, and adds that the acts of the martyrs may also be read on the anniversary of their deaths. Although this Synod decree does constitute a decision by a church body about what constitutes the canon, it was a regional body and wasn’t making the decision for everyone. It’s also clear that the acts of the saints often had a kind of quasi-canonical status, as they were also read in worship on an assigned day.
Starting in the 4th century, we start to see Bibles published that have the 27 books we think of as the canon. However, even into the 9th century, there are Bibles that are missing Hebrews or Revelation. So there is a good deal more variety than we imagine.
Not all Christians share the same canon. The Catholic and Protestant canons differ on the inclusion of the Apocrypha, for example. Other orthodox traditions have also included other books as part of their canons. So there is not a single, straightforward answer to this question.