During season 2 of The CW's The Flash, Central City celebrated its resident speedster with a special and elaborate celebration giving him the key to the city. But in the two seasons since that momentous occasion, there haven't been any more Flash Days and we can't help but wonder why.
During the episode "The Man Who Saved Central City," Central City hosts Flash Day as a way to thank Flash for saving the city. Six months prior, the Flash saved the city from the singularity caused when Eddie (Rick Cosnett) sacrifices himself to defeat Eobard Thawne. As Eddie was Thawne's direct ancestor and Eddie had, at that point in his life, not had any children that would ultimately lead to Thawne, this act creates a paradox that turns into a wormhole that started consuming Central City. To stop it, Barry runs in the reverse direction while Firestorm -- then Martin Stein (Victor Garber) and Ronnie Raymond (Robbie Amell) -- lends a hand. However, Ronnie dies in the process. In the episode, the city designates a Flash Day in gratitude, but despite all of the things the Flash has saved Central City from since there hasn't been another Flash Day.
So what gives? While its most likely that Flash Day was really just a one-off celebration, there are other reasons why it hasn't been held since. The first, and possibly easiest explanation, is Flashpoint. At the end of the second season, Barry ran back into time once again, stopping Thawne from killing his mother. It leads to Barry getting his perfect life when season three starts -- a timeline that becomes known as Flashpoint. That perfect life isn't all it seems, and Barry ultimately goes back again to undo it, but some changes stuck -- a huge example of that being the Baby Sara becoming Baby John on Arrow. It's possible that Flashpoint also did away with Flash Day.
Another possibility is safety reasons. Fans might recall that the first Flash Day was interrupted by another meta, Atom Smasher, attacking the event at Zoom's behest. In fact, any time there is a large gathering of people and the possibility that a superhero may show up, the chances that a villain will attempt to either harm the crowd or go after said superhero increases. After the incident with Atom Smasher, it would make sense that Central City would decide to not have any additional celebrations, at least not on the scale of Flash Day.
It's also possible that mundane and practical reasons have limited Central City to just the one Flash Day. Putting together large public events like Flash Day are an expensive item on any city's budget. There's security costs, possible economic costs with streets and such shut down, the cost of the decorations (you know those Flash Day banners aren't cheap even in the Arrowverse) and other details to consider. It is probably simply not cost effective to host a Flash Day every year. After all, awarding the speedster with a plaque for his heroism at a press conference (as was done during this season's "The Trial of the Flash") is far cheaper than a full-on festival and considering the damages some of the Flash's heroics cause, that money is probably better spent on city infrastructure.
Besides, Barry's in jail. Probably not the best time for a new Flash Day.
The Flash airs Tuesdays at 8/7c on The CW.