It really is easy, the more formal the wedding in time and location, the more formal the attire to be worn.
We tend to a bit of bending the rules to suit the comfort level and desires of the bride and groom. The time of your ceremony does not necessarily have to dictate the attire. If you are marrying after 3 pm, pretty much anything can go. More important is to consider the style of the reception. If a ballroom, private club, or mansion are the venues of choice, well formal wear really is indicated.
If you are marrying after 5 pm and you are having a lavish wedding, a tuxedo is a must. A black tuxedo, white shirt, vest (if inclined, we tend to not like them, but it is really up to your personal style) and bow tie are the quintessentially chic groom ensemble for a formal, lavish, nighttime affair. Vests are a more slimming
option than cummerbunds, but stay away from bold patterns and bright colors. They can detract from the overall look.
A tuxedo can still work for less formal nighttime affairs, but why not switch out the bow tie for a nice, long silk tie. This look is still polished, but not as a formal. It is less stuffy and transforms a long tie into a dapper accessory. Tres chic and very fashion forward. Another great option if you are going for a more semi-formal approach is a very dark suit over a tone-on-tone shirt. Add a skinny silk tie and the look becomes more stylish and personalized.
No matter what your final decision, the groom and his gentlemen should look and feel their most comfortable best.