Please provide the book, chapter and verse.
Using specific wording in Luke Chapter 1, we can actually determine that December 25th is the day of Christ's birthday; the problem with this is that it relies on a source outside of the Bible itself, so I'm not sure if the Sola Scriptura people are on board with this. (It's basically tying Scripture with Jewish history that was not recorded in Scripture.)
This is an excellent summary here: www.dec25th.info
https://www.youtube.com/watch?v=ptlsXtTf6n0
"Scripture is the perfect and only standard of spiritual truth, revealing infallibly all that we must believe in order to be saved and all that we must do in order to glorify God. That—no more, no less—is what sola Scriptura means." (link)
Use of Extra-biblical sources to infer the time of Jesus' birth do not violate the above definition of sola Scriptura.