Go to the Administration tab on the left side of your course screen.
Scroll down in the Administration Table to Gradebook Setup.
When the Gradebook Set up choose the item whose grade you wish to hide and click "Edit" in the Action column. This will give you a drop down menu, where you should choose "Edit settings."
This will open out editing options for that grade item. Open out more options by clicking on "show more."
The expanded options include a tick box for "hidden" and for enabling a date. Tick both boxes and choose a date after which the grade should be revealed, then click "save changes." The grade will now be hidden from participants until the date set. NB the student will see the grade item, or the assessment activity in the course and in their User Report, but they will not see the mark allocated until the date set.