Rather than asking our HR colleagues here, shall I suggest you that please write it on your own style and vocabulary so that you feel the pleasure of inviting from your heart and employees knows that the invitation is from your heart than a copy paste template.
The invitation should contain the below points.
Ethical dress code on the day of celebration
Celebration Date & Time
Celebration venue (floor or any space)
Few highlights of the celebration (Thank you note by MD, any cultural activities.....)
The beginning of the email, may start with thanking all the employees for their outstanding contribution and it's time to have fun and celebrate before we kick start a positive year 2018.
Add a small, new year wishes image from google.
"we wish you a Merry Christmas full of joy and a happy, healthy and prosperous 2018, together with your families and loved ones."