??? My First Classical Concert,What Can I Expect ???

Hello Freediver, first let me congratulate you on the decision to expand your live concert genre.  One suggestion is to avoid "overthinking" this new experience.  The most notable and most obvious contrast will be that you'll hear the purity of the artists,the instruments and the music.   No electronic enhancements such as amps, equalizers, mixers, dynamic lighting, speakers, etc. Also be prepared for a much lower SPL than you've encountered at rock concerts where the SPL averages 90 to 120 dBs.  Live classic concerts SPL averages around 70-90dBs (much less stressful on the ears) with a few loud passages approaching 100 dBs.  I enjoy every musical genre but especially so when the musicians are truly talented.  Arrive early and take you seat so that you can listen to the artists warm up.  This will also help prepare you for a marvelous first time experience.  Enjoy the music.

WOW,I debated starting this thread,not expecting the kind of responses posted so far..I’m really glad I did..I appreciate the informative replies so far..
 Just to add a few thoughts..I’ve been to plenty of small Jazz clubs, seen plenty of unamplified folk musicians & the occasional Classical Trio at weddings so it’s not like I’m unfamiliar with unamplified music..Just never been to a larger venue like this specifically featuring unamplified music...
 I would also note,I am a fan of most genres of Classical music & adore harp music(which my late wife played),wind instruments & the cello so this isn’t a complete venture into the unknown..
 Lastly & specifically for @horn13, could you elaborate on why you suggested this?

And if the piece is a real drum crasher you will gain an enhanced appreciation of what drums can do in a hall.

Pay attention to the sound and how it is affected by the space you are in. Notice the blending of the sections, and for goodness sake, don’t applaud between movements of the same piece (unless the performance was exquisite).

And have fun

This is mainly to reinforce what others have already said!

The six Brandenburg Concertos are very early 'classical' pieces and do not feature soloists like later concertos do.  The original meaning of concerto was 'playing together'.  The original instrumentation did not use any drums, instead the harpsicord provides an improvised rhythm section and according to Sir Thomas Beecham sounds like two skeletons copulating on a tin roof.  The volume produced by each harpsicord keystroke is fixed!

Each concerto is very short - somewhere between 10 and 20 minutes - but much longer than most modern 'songs'.  The first concerto has four individual movements with pauses between them, the other five are just three movements.  These pauses are the best time to wriggle, cough and say a few words to your partner.  Otherwise, try to stay silent.  Take a few cough lozenges unless you fancy being a featured soloist - always embarrassing. Best to wait for most of the audience to clap before you join in, unless you know the piece being played.

As far as acoustics go, in Bach's day the only venues were echo-prone churches and cathedrals, or much smaller rooms.  The Brandenburgs were composed for intimate spaces where the very fast notes don't get lost in reverberation.  You have to fast forward to Mozart's time to get custom built 'listening spaces' aka chambers and true chamber music.

The sound will vary dramatically throughout the church.  Being near the front, you may just be able to shut your eyes and localise an instrument, but the original intent was to play together!  I have never been able to pinpoint an instrument in a full symphony orchestra playing live in a real venue.

I hope you do have fun, but it can be hard when everybody around you is po-faced and deadly serious!