You do not mention which Saint Thomas you are referring to. There are several. Saints days are days set aside by the Church to commemorate the lives of those special people who gave their lives to God by serving His Church on earth.
The Day of St. Thomas, is celebrated on July 3.
Saint Patrick's Day
Saint Nicholas Day is celebrated world-wide on December 6.
The feast of Saint Patrick is celebrated on March 17.
Saint David's Day is celebrated on the first day of March.
Saint Patrick's Day was first celebrated in America in 1737 Boston, Massachusetts.
The feast day of Saint George is on April 23.
Saint Sebastian's feast day is celebrated on the 20th January each year.Saint Sebastian's feast day is celebrated on the 20th January each year.
Saint Patrick's Day is widely celebrated in the USA.
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Saint Lucia's day is celebrated on December thirteenth each year. It is celebrated in the Caribbean nation of Saint Lucia.
Most feast days are celebrated with a Mass in honor of that saint. If you want to know about local customs, you need to be more specific, as in, "How is St. David's feast day celebrated in Wales?"