Dear Felix
We have a request to ask of you.
Our office is having a party on [DATE, ex. next Wednesday] to celebrate [STATE OCCASION, our fifth anniversary of doing business]. I would very much like for you {and your wife} to attend; it would be our pleasure to have you.
[STATE TIME, LOCATION AND OTHER RELEVANT INFORMATION, ex. The party will be held at the French Inn, located at the north-west corner of First and Second streets. Refreshments will be served at 6:00 PM and dinner will be at 8:00.]
I hope you will be able to attend. [MENTION REQUESTED NATURE OF RESPONSE, ex. Please relay your reply to me at your earliest convenience.]
Regards
Lydia