Yes. A Catholic may attend an Orthodox Divine Liturgy — it is valid, reverent, apostolic worship offered by a true priest. There is nothing illicit about being present, praying, and adoring God in an Orthodox church; the Catechism affirms that these Churches “possess true sacraments” through apostolic succession. Two precise questions remain, and they have precise answers: does it count for your Sunday duty, and may you receive Communion?

The Sunday obligation. Here the exact wording of the law matters. Canon 1248 says the obligation is satisfied by assisting at a Mass “in a Catholic rite.” The trap is one word: “Eastern rite” is not the same as “Eastern Orthodox.” An Eastern Catholic liturgy — Ukrainian, Melkite, Maronite — fulfills your obligation, because it is Catholic. An Eastern Orthodox liturgy does not, because those Churches are not in communion with Rome. So attend freely — but as a supplement to your Sunday Mass, never a substitute for it.

Receiving Communion. Catholic law does make one narrow provision. Canon 844 permits a Catholic who cannot reach a Catholic minister to receive the Eucharist, Penance, or Anointing from a minister whose Church has valid sacraments — the Orthodox — but only “whenever necessity requires it or true spiritual advantage suggests it, and provided that danger of error or of indifferentism is avoided.” That is a high bar for genuine necessity, not a license for convenience or an ecumenical gesture.

And the honest bottom line is practical. The Orthodox Churches, by their own discipline, practice closed communion — for them, to share the chalice is to declare a unity of faith that already fully exists, not a means of pursuing one not yet reached. So an Orthodox priest will almost always decline to give the Eucharist to a Catholic. So the Catholic permission is one-directional and, in practice, usually moot. The right thing — and the gracious thing — is to attend with reverence, honor the host Church’s discipline, and not present yourself for Communion as a matter of course. Catholic law sometimes permits it; Orthodox practice almost always prevents it.