Como dizer "Trabalho interno" em inglês

According to FEBRABAN, tomorrow, December 31, although bank employees will be at work, banks will not be open to the public.

I can't think of any expression exactly equivalent to "trabalho interno" in this context. There is a set of expressions -- the front end and back end of a business -- but I wouldn't use them here.

Administrative activity = expediente interno.

Trabalho interno is "inside job" and we use that expression to explain some types of crime activities.

Did you heard what happen last night?
It was a brake in the local convenient store on the corner.
Some people are saying it was an inside job!

So, I am voting for "zumstein" because indeed his suggestion is the most used expression for "expediente interno" = Administrative activity

Yes, but not every inside worker is an administrator. Administrative work, in English, usually has a restricted meaning related to management duties.

Right, but an assistant of the Administrator can do administrative tasks under his supervision. Actually, a whole department can do administrative work under the Administrator and for that it can be interpreted as doing an "administrative work" or performing an "administrative activity".

Therefore, I think the word we are looking for here could be also "Administrative work"