There are many saints named William but the feast of St. William of Montevergine is on June 25.
Constantine is not recognized as a saint in the Catholic Church so has no feast day. He is recognized as a saint in the Orthodox Church, however.
Catholic Church, May 15th. Orthodox September 17th
January 20 (catholic) December 18 (Eastern Orthodox)
Saint Valentine of Rome originally had his feast day on February 14. However, in 1969 he was removed from the Calendar of Saints so no longer has a feast day in the Catholic Church. However, that continues to be his feast day in some Orthodox and Protestant denominations.
In the Catholic Church - September 21.In the Orthodox Church - November 16.
The two different churches simply chose different dates to honor the saint: Catholics on October 28 and the Orthodox on June 19.
In the Western World it is on 2nd January; in the Eastern Orthodox Calendar it is January 1st, our New Year's Day.
Pope Benedict I is not a saint and has no feast day.
Saint Lucy is mainly venerated in Catholic and Orthodox Christian traditions. She is celebrated on December 13th in honor of her feast day.
Saint Maria Goretti's Feast Day is July 6th.
The feast day of saint Mark is April 25.
Saint Laura's Feast Day is October the 19th.