The celebrations of the two saints you mentioned are memorials for most of the Western Church and are therefore trumped by Sundays which are equal to solemnities. However, for the religious orders to which these two saints belong, their celebrations are solemnities which equal Sundays. Therefore, they may celebrate them on Sundays in their houses and other entities that they staff. For example on August 8th, I celebrated Mass at Catholic Answers in honor of St. Dominic as a solemnity. However, certain celebrations of events in our Lord’d life take precedence over other solemnities. The celebration of St. Andre cannot take the place of the solemnity of the Epiphany.
This also holds true for the localities in which such saints lived or in places where they are special patrons. If the celebration of St. Francis of Assisi falls on a Sunday in San Francisco, it can be considered a solemnity.
Fr. Vincent Serpa, O.P.