Welcome to Santiago! There are a tremendous amount of things to see and things to do in Santiago de Compostela; you are encouraged to stay for at least one full day extra for exploring the web of streets, all of which seem to bring you back to the Cathedral.
Your pilgrim related business is likely to start in front of the cathedral, kilometer zero. A shell and plaque mark the spot in the center of Plaza Obradoiro (see below).
One word of caution regarding accommodation is in order. If you are arriving in the high season, you are advised to make a reservation in advance. There have been several additions to the albergue roster in recent year but the numbers of pilgrims still exceed capacity in the high season.
The Feast day of Saint James is celebrated with a full week of music and dance, with a fireworks display in the Plaza Obradoiro on the evening of the 24th of July. The best views can be had from Obradoiro, or from Alameda park.