That question has been answered today! YJ co-creator Greg Weisman answered this question on Twitter:
Too precise. All I know for sure is fourth quarter 2018.— Greg Weisman (@Greg_Weisman) December 28, 2017
Basically, fall 2018. That means the hype train can help it more until then. Trailers. Panels at conventions (SDCC anyone?). More teasers. About the only thing missing is a comic tie-in. Though Weisman did try to have that one done, it looks to have not worked out. Regardless, now we know when the new season will start.