Can you suggest a way I might celebrate one of Hephaestus's holidays/festivals, or just Hellenic holidays/festivals in general? Thank you!
Well, it really depends on the specific festival to be honest, and how you want to celebrate. I know of two festivals for Hephaestus, Hephaistia, and Khalkeia(which honored both Hephaestus and Athena as far as I’m aware).
Hephaistia, was a torch-race festival, and done to honor Hephaestus who grasped fire.
Khalkeia, was celebrated by craftsmen who worked with bronze. Bronzeworkers would begin work on a new peplos for the statue of Athena in Athens, that would be gifted to Athena on Panathenaea.
The standard kind of format for festivals looks like this:
Depending on the festival, sporting events, competitions, plays, mystery rites, and other events could be included during the course of the festival. My advice is to look at what you want to accomplish by celebrating the festival. Are you thanking a Theos for something specific? Are you celebrating the change of seasons or some passage of time? Are you using this festival to celebrate and prepare for one further down the road? Are you honoring the skills and domains of a specific Theos by celebrating that skill? The possibilities are endless.
Figure out what it is you want to accomplish, and you can build your celebration accordingly.