Dear Mr./Mrs. xxx
It is my honor to invite you to our xxxx (the name of the activity, such as "celebration of our tenth anniversary"). It will be held from xxx to xxx (time) on date/month/year at xx (name of the place. such as "our school"). Would you please confirm your participation before date/month/year by returning an mail or call at xxxxxxxx (telephone number)? The contact person is xxx(name of the contact person). Thank you very much and we are looking forward to your coming!
Best regards
xxx (name of inviter)
for your reference.