We don't know for sure yet if patch 2.4.2 is hitting the live servers tomorrow, but there's a few clues that may suggest we're going to see it soon. First is that maintenance tomorrow morning will be extended, from 3am-11am PST. Extended maintenance doesn't always mean a patch, sometimes it's for hardware upgrades or other reasons. However, if there's patch, there is generally extended maintenance.
But there is one other sign that could be a harbinger of 2.4.2. Tipster Phillip pointed out to us that the PTR servers are currently down. There's hasn't been an official notice posted on the US PTR Forums as of this writing, but there is an official notice of the end of testing on the European forums. Certainly, Blizzard's already had one false stop on the 2.4.2 PTR, but it could be that the second time is a charm.
If you need to jog your memory on 2.4.2's content, check out the initial patch notes here and the undocumented changes (most of which have since been folded into the official test server patch notes) here.