Die Frage, wann Vatertag ist, ist nicht so einfach zu beantworten. Koriander hatte sich schon mal darum bemüht, für den deutschsprachigen Raum Übersicht zu schaffen.

Die englischsprachige Wikipedia weiß aber wieder einmal mehr als ihr deutsches Pendant:

Almost all over the world, a Father's Day exists to honor and commemorate fathers or forefathers. In Christian/Catholic tradition, Father's Day is celebrated on St. Joseph's Day. Other cultures have different days to celebrate their fathers, and even in many traditionally Christian/Catholic countries a more 'secular/non-religious' Father's day is also held.
• Austria: second Sunday in June
• Australia and New Zealand: the first Sunday in September
• Argentina, Canada, Chile, France, Japan, the Netherlands, the United States, the United Kingdom, Malta: the third Sunday in June (June 19 in 2005)
• Belgium: St Joseph's day (which is March 19), and the second Sunday in June ("Secular")
• Brazil: the second Sunday in August
• Bulgaria: June 20
• Denmark: June 5, same day as Constitution Day
• Germany: on Ascension Day (originally intended by socialists to replace the Christian holiday, but now the two are generally combined)
• Korea, Republic of: May 8, Parents' Day.
• Lithuania: the first Sunday in June
• Republic of China (Taiwan): August 8
  The word for the date, ba ba, (8/8) is a homophone of the word for father in Mandarin Chinese, "爸爸" bàba.
• Scandinavia (Including Finland): the second Sunday in November
• Portugal, Spain, Italy: March 19
• Thailand: December 5, birthday of King Bhumibol Adulyadej

Und wie das so ist: die Gründe für den Vatertag sind auch alles andere als einheitlich.