Dear …

Thank you for the invitation to __(type of event)__. This is __(indeed, a milestone/quite an achievement/etc.)__ for __(Name of Organization)__ and I feel honored to be invited to the celebration. How thoughtful of __(you/Name/etc.)__ to include __(all the contractors/me/etc.)__ to be part of this wonderful occasion. __(I am/Both my guest and I are)__ very much looking forward to attending the __(dinner and dance/cookout/etc.)__.

Again, thank you for the invitation.

Sincerely yours,