you can apply now..all you have to do is visit the website of us embassy and fix an appointment and schedule an interview with an officer at US embassy in the first week of december or let say in seceond week I mean before chrismas right..and meanwhile you can ask your brother to prepare all documents and send them to you..once you received all documents only then schedule an interview..As far as occasion is concerned, It could be any occasion such as chrismas or new year, birthday pary, any party which is going to held in the US and most importantly your brother involves into it somehow. even you can go to US if there is no occasion at all..just mention during the interview that you want to see your brother..that all..this was what I replied when i was asked why do you want to go US by visa officer..
I would advise you to take an appointment in December if you plan to visit your brother on chrismas or new year...