हॅपी होली (Happy Holi)
It is pronounced as happi holi.
You can also use:
होलिका दहन की शुभकामनाएँ (Holika Dahan ki Shubhkamnaye) - This literally translates to "best wishes for Holika Dahan," referring to the bonfire ceremony that precedes Holi.
होलिका दहन की हार्दिक शुभकामनाएँ (Holika Dahan ki Haardik Shubhkamnaye) - Similar to the previous one, but with a more heartfelt meaning, "warmest wishes for Holika Dahan."
होली की हार्दिक शुभकामनाएँ (Holi ki Haardik Shubhkamnaye) - "Warmest wishes for Holi."