The anniversary of the apparition at Knock is on August 21, but most of Ireland honors Our Lady on August 17 in the liturgy.
Our Lady appeared in Knock, Ireland. on the night of August 21, 1879. Though she never said a word, her appearance comforted many and gave a renewed vigor to the faith of those of the Emerald Isle, and encouraged many to turn to the Virgin Mary.
The anniversary of this apparition is remembered each year at the Knock Shrineon August 21 with a special liturgy offered at the site of Mary’s appearance.
However, the rest of Ireland celebrates Our Lady of Knock on August 17. It is classified as a “memorial,” and parishes around Ireland may celebrate Our Lady’s apparition on that day.
Currently this celebration is only on select calendars, primarily in Ireland, and not on the universal Roman calendar.
The Facebook Page of the Knock Shrine explains that, “Although the Anniversary of the Apparition takes place on the 21st August (a day that we look forward to celebrating on Sunday), the 17th August was chosen as the Feast day of our Lady of Knock, as the 21st August is already assigned to St. Pius X, the Pope of the Eucharist.“
For the rest of Ireland (and most of the Roman Catholic world), August 21 remains dedicated to St. Pius X.
The rationale behind the switching of liturgical days is to give more focus to a particular feast day, instead of supplanting one that is already being widely celebrated.