Go through this link.... http://techtrainingnotes.blogspot.fr/2010/08/sharepoint-creating-calculated-column.html
=OR([Effective Date]=DATEVALUE("9/21/2015"),[Effective Date]=DATEVALUE("10/5/2015"),
=OR([Effective Date]=DATEVALUE("9/21/2015"),[Effective Date]=DATEVALUE("10/5/2015"),