The option period is a defined length of time that has been negotiated and agreed upon by both buyers and sellers. The option period starts the day AFTER the contract execution date and ends at 5 pm (local time) on the last day.
If you are buying a home in Spring Texas and you have a 10 day option period and the contract execution date is January 20th, then the option period ends at 5 pm (local time) on January 30th. All days in the contract are calendar days. Holiday or non-holiday days it doesn’t matter. Every day is counting as a day when determining the day that the option period ends.
But if you’re buying a home in Champaign Illinois, the option period may or may not end at 5 pm on January 30th because real estate is local and that includes the interpretation of when the option period ends.