Each congregation hymn has a prelude and many of them have a postlude. It is recommended that every organist try to learn the preludes and postludes as written and that they are consistent in playing them for each service.
from a letter dated August 17, 2012
For those hymns that do not have postludes, the organist should still "create" a postlude after the final verse of the hymn that will allow the congregation time to put their hymnals away. To make it consistent for the serving ministers, preludes and postludes should be played for each hymn sung by the congregation including the Hymn of Repentance and Holy Communion.
We recognize that some of the preludes and postludes may be difficult to play so it is acceptable to play a simpler prelude - similar to what has been done in prior years - until the published ones are mastered. The most important point though is that there is always a prelude played before each hymn and a postlude after each hymn.
NOTE: Many of the preludes and postludes in the new organist hymnal are longer than we have been traditionally used to. Please keep this in mind and allow the organists the time to properly play the preludes and postludes and eventually everyone will become accustomed to them.