Dear Ms. Kelly,
I'm Li Hua, Chairman of the School Student Union. I'm writing to invite you to be a judge at our English speech contest to be held in our school on September 2. As scheduled previously, it will start at 2:30 pm at the Lecture Hall of our library, and ten students will deliver their speeches on the given topic ― Chinese Folk Customs. It is expected that the contest will come to an end at 5:30 pm. I'd appreciate it if you could accept our invitation at your convenience. If possible, please call me at 88539688. I am looking forward to your early reply.
With best wishes!