City Hall gives you a wedding permit which has a valid date for 90 days, I think. Which means you can use this paper to get married before the 90 days is up. But it should also have a date on it, you can fill out which day you intend to get married. (On your wedding date the officient fills that form out for you, and you have 2 witnesses to sign the form as well.)
In my opinion, you need to first set an actual date for the wedding. A vague date sometime in Sept./Oct. is not good enough to convince the visa officer that you are seriously planning to get married. (Many young couples here plan their wedding one year ahead.) Once you have a firm date, you can go get your wedding invitation cards printed out, either by a company/store or yourself. My suggestion is to make it look as professional as possible. Then attach an invitation card and a photo copy of your wedding permit if you already have it, with the date of the wedding same as the one on your invitation card.