Obi-Wan once described a lightsaber by saying, "Not as clumsy or random as a blaster. An elegant weapon... for a more civilized age." The Jedi Master also failed to mention that, over the course of the saga, Lucasfilm wouldn't be able to figure out which button on Anakin's lightsaber actually turns it on.
Over at StackExchange, one user compiled a variety of images to showcase the different buttons on the lightsaber that have been used to turn it on. As you can see above, Luke ignited it by using a large rectangular button.
With this being the first time audiences got to see a lightsaber on screen, it seemed obvious that this was the only button that could activate the elegant weapon. Most toys that are modeled after Anakin's lightsaber also used this button to activate various lights and sounds over the years.
Throughout much of The Empire Strikes Back, we don't see close-up shots of Luke igniting the lightsaber, with audiences assuming it was activated in the same way as seen in the previous movie. Fans wouldn't see Anakin's saber on screen again until The Force Awakens, which ignited questions about the lightsaber's construction.
As you can see above, rather than the large rectangular button, Finn is able to turn it on with a smaller circular button. Additionally, an episode of Star Wars: The Clone Wars showed Padme igniting the weapon with the same button.
Whether these were merely oversights over the continuity surrounding which button ignites the lightsaber or if both buttons technically work is up for debate, as Lucasfilm is often quite good at maintaining continuity for even small details.
The mystery about which button turns Anakin's lightsaber on is only one of many recent debates surrounding lightsabers, with another being whether or not Luke will wield a lightsaber in The Last Jedi and, if he does, whose it will be.
Fans last saw Luke wielding a lightsaber he constructed himself in Return of the Jedi, which features a green blade. In one poster for The Last Jedi, Luke is seen wielding a blue lightsaber. It's unclear if this is a clue that he will, at some point, use his father's lightsaber once again or if he has constructed another lightsaber while in isolation on Ahch-To.
We'll find out more about Luke's lightsaber when The Last Jedi hits theaters on December 15.