Parts of the patch (251 MB for me) are now being sent out to the background downloader. This means that some of the patch data is finalized, and we can get those parts of the patch now instead of having to get everything on patch day.
Patch 2.3 was on the background downloader for 12 days [...]

(src) Later today we will begin providing most of the remaining data from our upcoming patch for players to background download. This second phase will be very large (approximately 450MB) as it contains the data for all of the upcoming changes as well as art assets needed in preparation for the release of the expansion.
This [...]