When we cannot fulfil our obligation to attend Mass on a Sunday or Holy Day of Obligation, whether because of illness or bad weather, or some other reason outside of our control, we can, and indeed should make an Act of Spiritual Communion. It is also good to make an Act of Spiritual Communion when we can attend Mass but something prevents us from receiving sacramental Communion that day—say, a mortal sin that we know we have not had the opportunity to confess yet.

We can make an act multiple times a day—even on days when we have been able to receive the Eucharist; because each Act of Spiritual Communion that we make increases our desire to receive sacramental Communion, and also helps us to avoid the sins that would make us unable to receive Communion worthily.