Yes, Father’s Day is capitalized when referring to the holiday.
Reason #1: While Father’s Day is not an official federal holiday, it is considered to be one of the most common celebrations, like Groundhog Day and April Fools’ Day. That’s why it is always capitalized.
Alternatively, the following words like father, mother, and dad are broadly termed as ‘family names.’ They are generally used as a common noun. However, whenever you are using these family names in the form of proper nouns, they need to be capitalized. For your reference, the examples are given below:
I) My father is not well. – Here the word ‘father’ has been used as a common noun and it isn’t capitalized.
II) I have planned to celebrate Father’s Day on 18th June. – Here the word ‘father’ has been used as a proper noun. Thus, it is capitalized.
Reason #2: Father’s Day is widely celebrated to honor fathers as individuals. This is the day when the children portray their sincere respect and gratitude towards their fathers. This day is special and unique to every father. Considering its significance and importance, the term Father’s Day is capitalized.
For more Father’s Day grammar, try the Grammarly Father’s Day quiz: