The date dimension is a calendar table that contains one row per day with its attributes: year, quarter, month, week, day of the week, public holiday, fiscal year. Facts reference it through a date key, which simplifies time-based grouping and avoids recomputing these attributes in every query.
Example
SELECT annee, mois, SUM(montant) FROM fait_ventes f JOIN dim_date d USING (date_key) GROUP BY 1, 2;