The day of the wedding, there are two important events (important at least for the invited guest, there are plenty of other important events for the couple doing the marrying) : the mass, and the party/reception.
Remember that we are in Mexico, the invitees can number in the hundreds and also, everyone is catholic which explains a lot of things about the state the country is in, but again I am digressing and that is definitely a subject for Christopher Hitchens, Sam Harris and Richard Dawkins.
So, we have a whole lot of people and a catholic church service. This is the case at least 94% of the time; I have been to a wedding - and fairly recently - where the religious portion of the marriage was performed by a Mayan X'men, or holy man, in an elaborate ritual complete with conch blowing, branches brushed all over the bodies of the participants to ward off evil spirits, lots of candles and plenty of smoke. All this on the beach in Uaymitun. Really quite different and as completely believable as any religious ceremony I have ever been to. Which, if you know how I feel about these things, says a lot.
Back to the subject at hand: for most weddings, you can expect that less than half of your guests will bother showing up for mass. While this part of the day is important for the bride and groom on occasion but more often than not, important for the parents, it is completely irrelevant to most of the people who are looking forward to a great party. So if the church isn't full, this isn't cause for concern. Depending on the priest doing the spiel, which in turn is a reflection on the amount of money invested in the eccliastical department, the quality of the mass will be either boring and ho-hum or interesting with lots of personal asides to the bride and groom and the parents. And your seating arrangements are not indicative of your relationship to the couple, which means you can sit wherever your heart desires. Don't forget to bring along a few coins or a small bill, cuz that basket does get passed at one point. you know the church needs the money, I mean look at the condition of the Vatican these days! And, be prepared (and this can be frightening if you don't understand spanish or the hypocrisy of catholicism) because all of a sudden people will turn to each other - and you - and wish you peace. You will not speak to these people again once out of church.
The music at the church is also important and again, the quality is directly proportional to your investment. An off-key violin trio and a shaky soprano really reaching for those high notes can make the most well-intended sermon go incredibly and horribly wrong. If you are lucky, you will be invited to a wedding like the one last night; the father of the bride had the taste and pesos to hire professionals: a small choral ensemble and orchestra, that played everything from Gloria (Haendel or Vivaldi?) to Morricone's theme music from The Mission. Perfectly in tune and an absolutely perfect way to ignore the silly man at the front of the church in his frock, changing hats - from the tall cream-colored bishop model to the more casual holy red beanie - every time he moves from a standing to a sitting position, all the while making half-hearted attempts at humor and to present himself as the consummate humble servant of the lord. Indeed. In any case, the musical selections were perfect. Music in Merida's churches has come a long way since Los Violines de Waldemar, I am happy to report.
After the whole ceremony is over, everyone showers the bride and groom with best wishes, hugs and kisses and then, it's off to the next part, where they will meet up with the rest of the guests who somehow 'couldn't make it' to the church. If the church is near the reception you can walk there, unless of course you are local which means you will probably get in your car, drive half a block and park it again, so that it's close when you leave the reception later.