The first Thrawn, the first R2 and the CLS event weren't listed in the initial monthly calendar either. They were all announced AFTER the first of the month. Part of it probably has to do with the fact that they don't want to guarantee a particular release date until they are certain it's ready to release...otherwise, the forums would be flooded with tears about how they "promised" us a particular release date and then didn't deliver. With this Thrawn event though, keep in mind that he's tied into Phoenix and the incoming Phoenix upgrades....so they probably needed to ensure that the Phoenix upgrades were ready for release before they could announce an official date for Thrawn's return.
The reality is that they gave an obvious hint with the Sept. 20th content update that Phoenix was getting upgraded and Thrawn was returning. The what's ahead section said: "A team rises from the ashes stronger as a familiar foe returns." That was a clear reference to Phoenix getting an upgrade and Thrawn returning. So this was "communicated" to you and everyone else. That's the point of the hints....they can't officially announce re-works and events that may be subject to modification or don't have firm release dates. So they hint at what's coming next so people can start preparing for it. In this case, we were basically told 2 weeks ago that Thrawn was on his way back.