Dear Sally,
We (XXX) would love if you can come to XXX's birthday party at 地点,时间(at - 几点,on - 星期几,日期／月份,例：at 4pm,Saturday,23rd of November).We will (活动内容,例：do some Kareoke then head to a bar).
It'll be a lot of fun,hope you can make it!
XXX