って can be used in place of は as the topic particle in casual contexts.
もの is 物. さ is just some filler sentence ender.
ご and お are sometimes used as a prefix to make something more polite. Sometimes it’s even so common for a word that it basically becomes the word, like ご飯.
Yep.
Maybe it’s 一等? I’m not sure.
See @Kazzeon’s answer for this.
される is the passive of する. されない is the negative of される. なくてはならない is a grammar point meaning you must do something.
Here かん is 間, so it’s a duration of 20 days. The も adds some emphasis, like it’s held for even 20 days.
はつかかん = for a period of 20 days (かん is added to time expressions to show duration)
も = particle to emphasise the extent of something (in this case the duration of the festival)
つづく = to continue
もの = something, i.e. the festival is a thing which… (I believe this is right, someone may correct me on this use of もの)
らしい = I heard, or apparently
If you were to translate this sentence word for word into English, it might read something like: “Apparently that festival I went to the other day is something that goes on for twenty days,” where twenty in bold type conveys the lengthy duration of the festival.