Stop users picking a certain date

Olivia, you can use and customize something like the following:

OR
(
   Date_Field__c = DATE(2015,10,1),
   Date_Field__c = DATE(2015,12,10),
   Date_Field__c = DATE(2015,5,5),
   Date_Field__c = DATE(2015,11,9),
   Date_Field__c = DATE(2015,4,28)
)

The date withing the DATE function should be in the yyyy,mm,dd format.