The faithful are not obliged to believe in apparitions by Jesus or Mary known as private revelation. An example of this would be the private revelation of Mary to St. Juan Diego in Mexico.
On the other hand, the Feast of Our Lady of Guadalupe is a holy day of obligation in Mexico. How can the faithful be obliged to attend Mass in honor of an event which the faithful are not obliged to believe? By obliging them to attend Mass, does this not imply “you must believe in this apparition?”
Private revelations do impose an obligation the faithful, that much is true.
Liturgy is a whole other matter.
When the Church approves a particular revelation for faith expression (the faithful are authorized to believe in it) it means there is nothing contrary to Catholic faith or morals.
In liturgy, a solemnity is a feast of the highest rank. Patrons are often ranked as a solemnity. When the solemnity is also a holy day of obligation, there is a binding precept to attend Holy Mass under pain of mortal sin, with the usual exceptions. This is laid out as the First Precept of the Church (CCC 2041).
Given that Mass is Christocentric and that the private revelation is not binding, the faithful bound to attend mass can always be grateful to God and the Church for the honor given to Christ through Mary. In the case of Mexico, this is especially true, as this particular title has received papal approval on multiple occasions and the Blessed Virgin is honored in a special way under the title “Our Lady of Guadalupe” as patroness in the region:
Pope Benedict XIV, in the Papal bull Non Est Equidem of 25 May 1754, declared the Blessed Virgin Mary, under the appellative of “Our Lady of Guadalupe”, as patroness of the Central and Northern American region.
Pope Pius XII accorded her the title “Queen of Mexico and Empress of the Americas” in 1945.
Pope John Paul II, at the request of the Special Assembly for the Americas of the Synod of Bishops, reiterated her title of Patroness of the Americas on 22 January 1999 and granted the rank of solemnity in that region.
Be as it may, remember there is only one B.V.M. and attending Mass on Holy Days of Obligation is a matter of obedience to the Church for the salvation of our soul.
There’s a bit of distinction here: OLG is an apparition, not (just) a private revelation. In that respect, there’s a little more variation in how the Church approaches the topic. For approved apparitions, the general consensus of the people is that this real event occurred and the Church’s investigation finds nothing in conflict with the deposit of faith. Therefore, the local Church (diocese or conference of bishops) can do something like making that feast day obligatory. You still don’t have to believe the full story, but you are encouraged to be open to the possibility, and more importantly, consider the message.