Dear Ms. Shrithi,
You haven't mentioned whether the ethnic wear is for Diwali day or the preceding day. However, the letter(s) that you're looking for are available in thousands of formats uploaded by our Cite HRians. You may use the "Find Info" facility at the top to get the necessary correspondence for your benefit.
Sample Invitation Letter for Ethnic Wear Day
Dear Colleagues,
Happy Diwali in advance to all of you! It is with great pleasure that I would like to inform you that we are organizing an "Ethnic Wear Day" on ________ as part of the Diwali occasion, to lift the spirits of this festival season. [Company Name] is excited to celebrate this day with lots of joy, events, and many more.
Looking forward to your participation.
With warm regards,
XXXX
Hope this serves your needs.