This month is the celebration of the founding of our organization. It is a big event, which will be marked by various group, and social events including a grand party, which will be the culminating event for the whole celebration.
It is my honor to invite you to our grand party. The date, time and venue are already printed on the party invite enclosed with this letter, as will as a program sheet for the occasion. I would like it very much if you could come but if there is any avoidable circumstances that will prevent you from coming, please give me a case at once.
The party will be a semi formal event so you can come wearing your casuals. We also have some accommodations ready so do not worry about it. I can also meet you if you want.