When one or both married individuals enter the Catholic Church, sometimes it is necessary for the couple to have their marriage “convalidated” — or, “blessed” — in order to be recognized as a marriage by the church.
Within the Church, there are many kinds of synods/councils, and to illustrate those it’s important to first briefly outline the organizational structure of Catholicism.
While anyone, Catholic or not, are permitted and encouraged to attend Mass as often as they like, non-Catholic Christians are not permitted to receive the Eucharist under nearly every circumstance, and non-Christians are not permitted to receive at all.