Dear Trade Growth,
Please understand that writing a letter in NO rocket science. Maintain a casual attitude while greeting colleagues.
May you like to refer the below sample:
Dear All,
Greetings from HR Department!!!
May the lights and sparkles of happiness get you path towards Progress & Continued Success. __________(company name) wishes you and your family a happy and prosperous "DIWALI"...!!!
(You can also add some picture to make your email look more elagant.