Hello, Navraj! I think the visa officers do not take into consideration the dates for certain parties or celebrations. In other words, they will not issue a visa before September 9 just because in the letter you mentioned about wanting your uncle to attend a September 9 party
The processing time varies so it will depend on how strong your documents is. In my opinion, the request length of stay is too long. People here in the forum would say that a maximum length of visit to request for a tourist is 3 weeks for a better chance of approval.
Either way, good luck on your uncle's application. Keep the faith.