Wait to post engagement "congratulations"
If the happy couple hasn't made their engagement public yet, don't be the one to spill the beans. There could be a number of reasons they haven't posted it on Facebook or other social media sites, so wait until they share the news to make your shout-outs.
RSVP as soon as possible
Not only is it rude to keep the wedding planners waiting, a late RSVP can also cost the couple a lot of money. If you have to cancel or change your RSVP, don't wait until the week of the wedding. The family may have someone else they would like to invite in your place.
Arrive on time
You don't want to be the person sneaking into the chapel in the middle of the ceremony. Not only is it rude, your late arrival may detract attention from the ceremony.
Put your phone away during the ceremony
Leave the photography to the professional. You can take pictures of the bride and groom during the reception. Even if the couple asks guests to take pictures for Instagram or Facebook, be courteous. That means make sure your flash is off if you decide to take a picture of the "I dos."
Bring a gift
Even if you don't know the couple well, if you go to the wedding or reception, be sure to bring a gift. It doesn't have to be big, just something practical that they will appreciate. Picture frames and champagne glasses are always good choices. See this post from Good Housekeeping to find more about what occasions require gifts.
Sit in your assigned spot
Couples often spend uncountable hours planning the seating arrangements for the reception. Don't throw a wrench in the plans just because you want to sit near an old college buddy. There will be plenty of time for socializing after the meal is over. If you don't know anyone at your table, don't worry! Get into the spirit and make new friends.