The 1st appearance of the Bat-Signal was technically in Detective Comics #60 but it’s turned on for the first time in “Tomorrow,” a back-up story for Batman #0.
The story takes place when Batman is just starting out. He’s more myth than man. Commissioner Gordon conceives of the Bat-Signal not simply as means to contact the vigilante, or even to intimidate the Gotham underworld, but to inspire the next generation of heroes. “We have to let the heroes of tomorrow know that they aren’t alone,” Gordon tells his daughter Barbara, who is destined to become Batgirl. The following pages feature the daily lives of the three future Robins: Dick Grayson, Jason Todd and Tim Drake. It ends with all of them looking up in awe at the Bat-Signal.
It fleshes out a world where there is a tendency to think these characters just popped into existence with no overlap or interaction with each other before their first appearance.