Great question - but an adequate response would be very complicated. The liturgical reforms of Vatican II designated saints days as Solemnities (e.g. June 29, Saints Peter & Paul), Feasts (e.g. St. Mark, April 25), Memorials (e.g. Charles Lwanga, June 3) and Optional Memorials (e.g. St. Patrick, Mar 17 or St.Thomas Becket, Dec. 29). Other saints are celebrated locally like St. Felipe de Jesus (Feb 5 in Mexico)or Blessed Kateri Tekakwitha (July 14 in the U.S). Saints with limited universal interest or who had little testimony to their historicity like St. Christopher were dropped from the calendar. That of course was not a statement that they did not exist. The story of St. Christopher carrying travellers across the river and finally receiving the grace of carrying the Child Jesus still has great value in teaching Christian charity. To wear a St. Christopher medal is a testimony to that truth. The Cult of St. Christopher has a fascinating history.
St. George is included in the universal calendar (April 23). There is good testimony that he died as a martyr in Lydda in Palestine during the 3rd or 4th century. However, the popular story about him slaying a dragon and rescuing a king's daughter originated in 12th century Italy. Once again the story has value in illustrating a truth of our faith. St. Valentine lost out to Sts. Ciril and Methodius who are celebrated on Feb. 14 as patrons of Europe. However, for obvious reasons, St. Valentine continues to eclipse them in popular appeal. In his case we are probably talking about an historical person whose legends also could have historical (as well as catechetical) value.
In teaching the Christian faith, we need to do our best to distinguish between historical and legendary because our faith is rooted in an historical event - the Incarnation. In this sense we are very different from say Buddhism which could do just fine if it were proven there were no historical Buddha. As I say, this subject is a complicated one - but important because we not only profess belief in Jesus but the Communion of Saints.
Fr. Phil Bloom