How to set a Date in apex test class?

You can do this using Apex' Date class:

acc.effectiveDate__c = Date.newInstance(2016, 12, 9);

For more information check out the documentation: Apex Developer Guide - Date Class

Another option is using system.today() (or system.now() if it's datetime field):

acc.effectiveDate__c = System.today() + 5;