Dear Tom, 
I am writing this email to invite you to join Lucy’s birthday party. 
The day of October 20th will be Lucy’s l8th birthday. We’re going to hold a party in Lucy’s house this Saturday evening. Many of our good friends and classmates are coming. We will have a big dinner together. After that we will sing, dance and play games. The party will last for three hours, from 7 p.m. to 10 p.m. I really hope you can come and join us. Our friends and classmates will be very happy to have you with us. I'm sure we will have a lot of fun together. 
Please let me know whether you could come or not. I'm looking forward to your early reply. 
Best wishes, 
Li Ke