Hover your cursor over the assignment. Click on the Options tab that appears.
Expand the assignment unit to see the assignment you wish to hide until a specific time. Hover your cursor over the blue horizontal bar of your assignment. Then click on the Options tab that appears.
Click on the radio button Hide from students until date. To set a date, click in the calendar for when you'd like your assignment to be visible. You may also set a time, if you wish. Then click Save.