Atma thinks it's always the first sunday in June with is used to remind people at large to reflect on the goodness of ones father(s) and to use the day for honor and veneration. May all take this opputunity well and if still alive, of which is not for sure, still possible to personal approach, such is a blessing and not good if not used. Mudita
Actually, as just seen, the
father's day's 
date varies according to the country, and as for the tradition my person was used to grow up as child (Austria), it's also the secound sunday in June (14. June 2020), not the first.
Most usual is the third sunday in June (21. June 2020), as seen, so also in Cambodia, since a merely new costum here.
Such as a traditional father's day, in the land of the Khmer, would be celebrated on King's fathers birthday and maybe it does so, similar to Thailand.
Of course it's the best to remind, honor and do ones duties toward ones father, whether still living or already departed, every day, especially doing and sharing ones merits.