One step of my workflow needs to calculate how many BUSINESS days i have between two dates. The problem is that my customer registers the holidays in a sharepoint list. So i will need to query this list from the workflow. How can i achieve it?
You could use a SharePoint Calculated Column to do the work for you and then use that in the workflow.
This blog post has some info about working with weekdays in a calculated column: http://blog.pentalogic.net/2008/11/working-days-weekdays-holidays-sharepoint-calculated-columns/
The answer on this thread also has a formula that may work for you: http://social.msdn.microsoft.com/Forums/en-US/sharepointcustomization/thread/d6a6b014-c09c-4436-9f18-0f151bae85b5