Can I make a resource or an activity accessible only from a certain date?

Yes, of course! For each activity or resource you can decide under what conditions it will be visible to students; among these you can also set the date or a time interval. In order to set the date, you have to activate the modification of the resource (by clicking on Edit next to the resource in question), then in the Restrict access section click on Add restriction, then on Date and enter the exact day and time from which you want to make the activity or resource accessible to students. Finally click on Save and return to course.