Compared to early episodes, the Imperials are threatening to the point of being almost unrecognizable. The Stormtroopers who half-heartedly bullied citizens and frightened Wookie toddlers have been replaced with a more insidious enemy, one that controls society and could make you disappear at anytime. What's more, Rebels is doing a fantastic job of showing the danger, not just telling. "Empire Day" is filled with tense quiet moments like when Stormtroopers watch the Ghost crew without saying a word, or when fear grows on a Rodian's face when being racially profiled. And that's before the Inquisitor makes a surprise appearance (suddenly emerging from burning aircraft wreckage), and his presence was already terrifying in any non-finale episode. Rebels has pulled off the impressive feat of making the Empire scary when it isn't actively doing anything, even when the band plays a high-tempo Imperial March.
"Empire Day" brings the focus back to Ezra, but finally explores his backstory in a satisfying way. While light on details, it's slowly revealed that his parents were anti-Imperial sympathizers who disappeared when Ezra was seven years old. You don't need to squint to piece the dots together, but Ezra keeps quiet about it for long enough that it adds a strong air of mystery to what exactly happened. Throw in the fact that Tseebo himself knows what happened to them, and Ezra's orphan subplot is suddenly a lot more interesting.
What feels a little odd is that, even accounting for the increased Stormtrooper threat, the Ghost's crew isn't quite on its game. These are heroes who frequently deal out massive amounts of damage to entire Stormtrooper squadrons. But here they're missing blaster shots, getting blindsided, and generally being overwhelmed by fairly light resistance. I suspect that's because Rebels is saving a truly climactic confrontation for part two, but it's a little jarring. This is the crew that broke into a Star Destroyer during the premiere and blew up its docking bay on the way out. It certainly raises the episode's tension, since Ghost may not survive unscathed for once. But there's probably a better way to do that then making the crew seem less competent than before.
Regardless, this is a solid episode that goes a long way towards getting fans excited for the next chapter. If the second half can satisfactorily resolve the plot elements "Empire Day" established, this might just be the best Rebels storyline yet.
Bottom Line: "Empire Day" is a fantastic opening to the midseason finale, only suffering because act two isn't here to back it up. Everything from the pacing, dramatic tension, character developments, and early action set pieces are top notch, leaving you legitimately disappointed once "To Be Continued" crosses your TV screen. This is Rebels at its finest, and it's absolutely worth the watch.
Recommendation: If you're not already watching Star Wars: Rebels, this is a great episode to dive in for.