Fact: We see Vader’s helmet for the first time in A New Hope. Fact: We see Vader don his new headgear for the first time in Revenge of the Sith. Fact: This is the first time he wears the helmet – WRONG! We no have a canon origin for the helmet Vader would wear the rest of his life post Mustafar.
With the release of the new comic Star Wars Adventures #13, a younger reader friendly comic from IDW, we get the canon story of Anakin during the time of the clone wars, and how he comes to wear his future infamous helmet for the first time.
The story goes Padme and Anakin are to attend a play. Things turn south when the main actress attempts to kill both Padme and Anakin. It fails and they discover the actress is a Separatist, working for Dooku.
Dooku hopes to kill the audience, made largely of Republic politicians. The lead actress and her leading man are to don special costumes that will protect them from the toxins while simultaneously distracting the crowd from realizing they are getting gassed until it is too late. To stop the lead actress, Anakin must take the place of the costar to get on stage. In the process he wears a mask that he will get to know much more closely in a few years.
This could be just a fun use of the mask, but this comic is indeed canon despite being meant for younger readers. It could be a cute homage to the future, but it could fit as well. Sidious has his hands into everything and if Dooku is involved, perhaps Sidious was testing out the new gear for some special purpose. Do not forget that it was Sidious and Dooku that turned Grievous into the metal monster he became.
Check out Star Wars Adventures #13 for the full story.