The visitors center is open daily 10am to 6pm (between 15. March and 31st October).
On Sa and Su at 3pm there are also guided tours into the pit. But I guess only in German. You have to register in advance for such a tour. Duration is 2 hours.
Never been there since they built the visitors center a few years ago.
Note that the below brochure in English is outdated (e.g. on opening times).
You might want to contact VT member Trekki because she lives there. She may spot this and answer but you could VT-mail her.
Hi Cristian :-))
and thanks Sally :-) I spotted his question indeed :-)
Cristian: let me know if I should call them and make appointments for you. I send you mail with details.