Frankfurt to Interlaken

How? Train obviously.
First question, are you coming from Frankfurt-am-Main (city centre), Frankfurt-am-Main Airport/Flughafen, or Frankfurt-an-der-Oder (a totally different place at the other side of Germany)?
Second question, are you really going to Interlaken, or somewhere else further into the Berner Oberland (Lauterbrunnen, Wengen, Grindelwald. Mürren etc.)?
Go to the German railways website: https://www.bahn.com/en/view/index.shtml
Enter from: Whichever Frankfurt you mean, to: wherever, date and time, and it will give you times and prices.
Tickets are always cheaper bought in advance, but once bought cannot be exchanged for a different time. If you are flying into an airport on an intercontinental flight, it is not usually a good idea to commit to a specific train as your flight may be delayed.

Also note that you will not be able to see fares for October next year yet, and possibly not times. To get an idea of times and the discount advance fares, put in a date about three months in advance. German rail tickets go on sale 92 days before date of travel.

News: German rail tickets are now for sale up to 180 days in advance and this also applies to tickets they sell with a Swiss or Austrian connection, or the direct train from Munich to Venice.