"Prelude music is played as members gather before the meeting. Its purpose is to create an atmosphere of worship that invites the Spirit. After the closing prayer, an instrumental postlude is played as members leave the meeting. Hymns and other sacred music can be selected for prelude and postlude music." 

-  General Handbook: Serving in the Church of Jesus Christ of Latter-day Saints 19.3.2

Timing your prelude music is crucial because it ensures that the music fills the designated prelude period appropriately, neither ending too early nor running too long. The prelude sets the spiritual tone for the meeting, helping the congregation prepare in quiet reverence for worship. If it's too short, there may be an awkward silence before the meeting starts; if it's too long, it can interrupt the flow of the service. A well-timed prelude reflects good preparation, supports the spirit of the meeting, and shows respect for the Lord by worshipping Him intentionally.

The "General Handbook: Serving in the Church of Jesus Christ of Latter-day Saints" doesn't give any direction on a specified length for prelude and postlude. However, at least 15 minutes for prelude and 5 to 10 minutes for postlude is generally best.

• Watch the clock carefully – Time your selections so you finish just before or as the meeting begins.

• Choose a shorter piece – End with a short hymn or arrangement that you can wrap up quickly. 

• Use cadence points – Find musically satisfying places in the music to resolve to, rather than cutting off abruptly.

• Plan your ending ahead – Know where and how you'll stop, especially if you're in the middle of a longer piece when the meeting begins.

• Have clear communication with your leaders - It is important to communicate with your priesthood leaders so that you and they know what to expect. Many issues can be avoided or easily solved with clear and respectful communication.

• Don't panic – If the priesthood leader stands to begin the meeting, don't stress. Gracefully finish the best you can.

Section 19.3.2 of the "General Handbook: Serving in the Church of Jesus Christ of Latter-day Saints" states that, "Hymns, worshipful children's songs, and other sacred music can be selected for prelude and postlude music." There is a great wealth of music written for the organ that could serve appropriately as preludes and postludes for church services. The "Organ Hymn Arrangement" practice activity in this lesson can give you some ideas.