It occurs to me a couple of solutions:
First is load your table with that specific query passed on to your sql server, guessing (untested)
Table: LOAD Field1, Field2, Field3; SQL SELECT Date FROM Store_Information MINUS SELECT Date FROM Internet_Sales;
Second is create a map of those dates which do appear in the firs table, and using them as a filter to load all values apart from them coded kind of
ExceptionsMap: MAPPING LOAD Date, 1 AS Flag; SQL SELECT Date FROM Internet_Sales;
TableMinus: LOAD store_name, Sales, Date WHERE ApplyMap('ExceptionsMap', Date, 'No') = 'No'; SQL SELECT store_name, Sales, Date FROM Store_Information;
Of course, field names may vary, and I used your example to clarify. Hope that helps!
EDIT: Although both should work, I posted before reading Michael's answer, which is clearer and smarter.