Normally, the Cathedral at Santiago is open every day of the year. However, just now, it is essentially closed for all liturgical services due to renovation work that got in the way of holding services there. You can still get in to venerate the relics, but all Masses and confessions are moved, for the duration of the renovation work.
So, all Masses, including Pilgrim Masses have been moved to other nearby churches for the time being. The daily, noon Pilgrim Masses, and all important celebrations of the Mass (Feast Days, Holy Days, and Holidays like Easter) have been moved to the basilica of San Francis until the renovations are completed.
That is the very large church of Saint Francis located at the end of Rua de San Francisco. As you look at the Western Facade of the Cathedral, proceed down the street to the left of the Cathedral. Rua de San Francisco is to the right side of the Parador. You will see the huge statue of Saint Francis at the end of the street. The Basilica is behind the statute... can't miss it.
Also, the daily 11:00 non-Pilgrim's Mass is in the Parroquia de la Corticela. That is the ancient church in the northeast corner of the Cathedral, adjacent to the north transept exit to Plaza Inmaculada. If you have been there you might have noticed the church at the end of the corridor in this corner. That is the original church, built before the Cathedral was built around it.
Here is a link to the chart produced by the Cathedral to explain these changes. It is in Spanish. But you can easily figure it out.
Also, for those interested in the daily Mass in English, here is a notice from Fr. Manny. Essentially the daily Mass is at 09:30 in the Pilgrim Office chapel, at Rua das Carretas 33:
(See attached .pdf file)
Hope this helps.