Korea (as well as China and Vietnam) celebrate two New Years. One on Jan. 1 and the Lunar New Year celebration, which this year falls on February 18-20
For your Korean colleagues (in Korea), you can wish them
Happy Lunar New Year by phone, text, or email on Monday PM.
(Tuesday AM in Korea).
Koreans will have a 5-day weekend starting Wed K time.
For expat Koreans working outside Korea/ globally you can wish then
Happy Lunar New Year on the day itself, Thursday February 19.
Here is the formal greeting--Sae hae bok mani ba deu say yo
BTW Following Korean zodiac tradition this is the Year of the Blue Sheep or Ram. A year of contemplation and peace.