thx Julie, I created an additional weekend calendar to assign tasks to. However, the amount of work (e.g.) does not match with the size of the task bar, the length will always be full day work (8 h) even when I put the task to 0 or 2 hours. Why is that ?

Are you referring to the length of the bar in the Gantt chart? Try right clicking in the back ground of the Gantt chart and selecting Layout. Uncheck "round bars to whole days".