You HAVE to use something other than Firebase cloud functions as a trigger... whether that's a 3rd-party chron job service, that's up to you. This Firebase article addresses this issue and walks you through doing it with Google's App Engine... but whether you choose to use a Google-backed solution or a well known 3rd-party service is up to you.
The cron job should call/trigger your cloud function... and in your case, the cloud function should loop over all of the users in that database table and reset (or add 1 to) the "nombre_dons_day" field.