From the perspective of logic, a statement is something that can be determined to be true or false. Or put in other words:
A statement is an assertion that can be determined to be true or false.
It is not possible to determine if "July 4 was a Monday," as the year in reference is not provided. July 4 could be any day of the week depending on the year that it lies in. It is neither possible to say that this is true nor is it possible to say that it is false; this does not make it eligible to be called a statement.
July 4 was a Monday is not a statement as it cannot be determined if this assertion is true or false.