It's called "Fukuoka Castle Sakura Festival". There's around 1000 trees in the castle ruins and in the nearby park(s) - Maizuru Park is is connected to the castle grounds, but Ohori Park is also nearby. And at night, these trees all lit up! While traversing the castle ruins and the park is free during the daytime, at night there is a fee to get in and walk around; I believe it was around 300-400 yen.
My friend and I were staying near Hakata Station so we took a short ride over to the Ohori Park exit and from there, it's a short walk to the castle ruins. During the festival, there will be LOTS of people going to the same place so you really can't miss it.
Once you enter through the front of the castle there's a few different ways you can go. My friend and I eventually located a map, but we chose to just go where our feet took us, so we wandered around a lot.
There are also additional areas not normally open during the daytime that you can access at night for better viewing, for yet another small fee. You can pick and choose which areas to enter; one of my favorites was climbing to the top of the castle and getting a great view of Fukuoka.
On one edge of the park/ruins is a huge group of food stalls!
My friend and I eventually looped back around to where we first entered (it's quite a walk if you follow the recommended viewing path). And that is the Sakura Festival at Fukuoka Castle! I hope you enjoyed, and maybe try to catch it if you're in Fukuoka in the springtime.