Hi/Dear xx,
It's a great pleasure to write to you, and hope you are doing fine.
First, I would like to express my sincere thanks to your help.I feel so grateful for that. Nothing could have accomplished if without your help.Therefore, i want to invite you to my house on christmas. I am wondering if you are free or not at that time. Pls write to me and tell me what your plan is. I really hope you can come and assure you that we will definitely have a great time togther.
Looking forward to your early reply.
Thanks & Best Regards,
xxxx