The show is 50 minutes long (I think) and is performed twice on Saturday and twice on Sunday. The show is tremendous, but be prepared because it will bring huge crowds into
Disneyland itself. It takes place on Town Square with a big stage set up in front of the train station facing the castle. The show will delay the start of the fireworks each evening by 15-20 minutes if memory serves me correctly.
Seating is extremely difficult if you can't get dinner package tickets, which are pricey. The first show is always tougher to get seats for than the second, but both will require waiting a long time(90 minutes at least) to get your seat. Best bet is to wait for the second show and ask a CM for help on where to go to get seats for the 2nd performance shortly after the 1st one ends. Do this before the 1st performance.
That said, I loved the show and highly recommend it, though it may not work for younger children with short attention spans. Guests should know that the Candlelight Processional tells the story of the birth of Jesus Christ and features many beautiful performances of beloved Christmas hymns. It is a very religious show and does not focus at all on secular holiday traditions. The processional starts at Small World and follows the parade route down to Main Street, with the choir members holding candles and singing Christmas hymns about the birth of Jesus. A celebrity narrator will then read the Christmas story from the Bible interspersed with choir numbers and trumpeters from the roof of the train station.
I recommend that anyone of a Christian faith visiting Disneyland during the Processional do what they need to in order to experience this tremendous Disneyland tradition dating back to Walt's time. It is beautiful.