Q. 1. Are Catholic priests obligated to celebrate the Holy Mass everyday? On their day off, when they do not celebrate Holy Mass at the parish Church, where do they celebrate Mass?
A. 1. That answer is found in the Code of Canon Law where it states:
Canon Law # 904 "Remembering always that in the mystery of the eucharistic sacrifice the work of redemption is exercised continually, priests are to celebrate frequently; indeed, daily celebration is recommended earnestly since, even if the faithful cannot be present, it is the act of Christ and the Church in which priests fulfill their principal function."
It is recommended that priests celebrate Holy Mass on a daily basis, but such is not mandatory.
Where your parish priest celebrates his Holy Mass on his day off, that is a matter you have to take up with him. The answer can vary from priest to priest. The priest may be celebrating Holy Mass at a Convent, a detention centre for youth, a Hospital, a Prison, a School, a University, an army base, at another parish, at the airport, at a funeral, at a Wedding, etc... Because you do not see the priest celebrating Holy Mass, it does not mean that he is not doing it!