Final answer:
The Feast of Saints Peter and Paul on June 29th is a holy day for the universal Church but not in the U.S., reflecting the integration of Christian and pre-Christian holidays in the church calendar.
Step-by-step explanation:
The feast day of two saints that is considered a holy day for the universal Church but not for the Church in the U.S. is the Feast of Saints Peter and Paul. This feast day is celebrated on June 29th and is of great importance in many parts of the Christian world, although it is not a holy day of obligation in the United States.
The universal Church calendar often includes other saints' feast days that coincide with pre-Christian holidays, which were integrated to aid in the conversion and acclimatization of new believers to Christianity.