The Skullcap You See Some Churchmen Wearing
Q: Why do Catholic bishops wear yarlmukes? I thought that was just a Jewish custom. Did Christian men also used to wear yarlmukes and could one wear one at Mass today?
A: The skull cap that is worn by the pope and by bishops is not a yarlmuke (though it looks similar) but is known as a zuchetto.
While wearing head coverings was much more common in the past (when people spent much of their time working out of doors), it does not appear that there was ever a time when it was common for Catholic men to wear zuchettos, especially at Mass, since one of the distinctives of Christian worship is that (unlike in Judaism) head coverings are not to be worn by men, especially during certain portions of the worship service (cf. 2 Cor. 3:12-16, 1 Cor. 11:2-16). Some aspects of what Paul said are culturally relative to the significance of head covering in his day (which is not the same in ours), but during specific liturgical functions it is inappropriate for men to wear head coverings. For example, even bishops do not wear their zuchettos when saying the Eucharistic prayer in the Mass.
A layman wearing a zuchetto or yarlmuke would not be a problem if it did not give anybody the impression the man was an ecclesiastic (little danger of that so long as he doesn’t go around wearing other trappings of ecclesiastical office) and if it didn’t give the impression that he is a non-Christian Jew.
People might be extremely scandalized if a man wearing a zuchetto went up to receive Communion in an ordinary parish, thinking that a non-Christian had just been given the Body and Blood of Jesus. This could cause the priest who gave him Communion untold problems, and if the priest did not know the man to begin with, it could even cause confusion for the priest when someone approaches him for Communion while wearing what appeared to him to be a symbol indicating a non-Christian religious affiliation.
However, in other circumstances, this would not be the case. For example, at a Mass being said for a group of Hebrew Catholics, some of whom might choose to wear yarlmukes, there would be nothing at all wrong about it.