Dear ,
We treasure every opportunity to meet with you, our valued customer.
From 28 April - 1 May 20xx, we will be exhibiting at the "Hong Kong Gifts & Premium Fair 20xx".
We cordially invite you to visit our booth.
Our booth number is 3D11 in Hall 3.
You may click here to find our location.
With Compliments
(COMPANY NAME)
基本上就是这个格式了，当然你完全可以把它弄得漂亮些。