Boppard's wine fests (scroll down) take place on successive weekends near the end of September.
In 2019 they take place September 27-30 and October 4-6. Saturday is the biggest day and the night for fireworks - the order of events stay fairly consistent from year to year. Here's an online flyer in German with the 2016 schedule:
https://www.boppard-tourismus.de/files/weinfestboppard16_flyer.pdf
Upstream on the same river bank is the Oberwesel Weinmarkt (also a fest) held "...annually on the 2nd and 3rd weekends in September.
2019: 13th -16th and 20th - 21th of September"
https://www.oberwesel.de/no_cache/en/highlights/weinmarkt/
Mosel wine fests (scroll down to September) : https://www.mosel-weinfeste.de/mosel_weinfeste.htm
Transport is no problem here. See train line maps below for Rhine rail lines and train stops on both river banks.
https://www.mittelrheinbahn.de/assets/images/6/MRB_Karte_Streckennetz_2018-66058e06.svg
http://www.vias-online.de/data/9_netzplan_rhg.jpg
The Mosel railway line follows the river closely most of the way to Bullay. See map (not all stops shown) : https://de.wikipedia.org/wiki/Moselbahn#/media/File:Moselbahn.jpg
From there to Trier, the railway takes an easier land route away from the river. Getting to Traben-Trarbach is done via an offshoot train line from Bullay. River towns without rail stations (like Bernkastel) are accessible by bus connection from Traben-Trarbach, Bullay, Wittlich, or other rail stations. The German Railways itinerary page provides these connections and of course all the train schedules:
https://reiseauskunft.bahn.de//bin/query.exe/en
Find the Mosel line and its stops (and the Rhine as well) on this larger map of the regional train lines:
http://www.vrminfo.de/fileadmin/data/pdf/2017/RLP-Ticket_streckennetz.pdf
Mosel cruises to non-rail towns are possible here too:
http://www.moselpersonenschifffahrt.de/downloads/Prospekt.pdf