Why use a Date Dimension Table in a Data Warehouse.
The Date dimension is one of these dimension tables related to the Fact. Here is a simple Data Diagram for a Data Mart of Internet Sales information for the Adventure Works DW database which can be obtained for free from CodePlex or other online sources.

The relationship is created by the surrogate keys columns (integer data type) rather than the date data type.
The query users have to write against a Data Mart are much simpler than against a transaction database. There are less joins because of the one to many relationships between the fact dimension table(s). The dimension tables are confusing to someone who has been normalizing databases as a career. The dimension is a flattened or de-normalized table. This creates cases of duplicate data, but the simplistic query overrides the duplicate data in a dimensional model.
Reference:
https://www.mssqltips.com/sqlservertip/3117/defining-role-playing-dimensions-for-sql-server- analysis-services/
https://community.idera.com/database-tools/blog/b/community_blog/posts/why-use-a-date- dimension-table-in-a-data-warehouse