Dressing up for the holidays in SP would be a clean shirt (with a collar for a more formal look).
Get everything at the local grocery, don't bother bringing anything unless it's some special gourmet product you can't live without. We've been over Christmas three times. Most stores and restaurants are open on Christmas Day to some extent. The real SERIOUS holiday in SP is Easter, which lasts for 4 days. The entire village virtually closes down for Easter Monday.