The first thing is obvious: these singings last only 2 hours or so, rather than all day, and they occur monthly rather than annually. Usually those who attend are drawn from the immediate geographic area, rather than from all over as for the big annual all-day singings. Loaner books are always available at these singings, so visitors and the merely curious may drop in and find out what the singing is like without making an investment in a book (about $15).
--The monthly singings often employ both the Denson edition and the Cooper edition of The Sacred Harp (with loaners for both available). Occasionally other shapenote books will be sung from. Monthly singings are also the place for local composers to try out their compositions with the class.
-- Monthly singings are often called "practice singings", as they are much less formal and the pace can be (though it doesn't have to be) much slower than at the all-day singings. If the trebles, for example, need to sing their part to a given song twice, time is often taken at the monthly singings to let them do that. Sometimes it's hard to distinguish a "practice singing" from a singing school, and at other times everyone just jells and the afternoon or evening feels like a big all-day singing in intensity.
-- Unlike the all-day singings, these events are not opened and closed with prayer, and most of the time the numbers of singers are few enough that leaders lead from their seats rather than standing.
Because those with the skill to pitch a song without mechanical aids are few, and there are lots of monthly singings, the pitching is often done with the judicious use of pitch pipes or tuning forks (guided by the same sense of where the key ought to be, however, as is employed at the all-day singings).
-- A very few of the monthly singings here in the Pacific Northwest have no structure in who chooses songs; most employ some technique such as "going around the square clockwise" to ensure that everyone present gets a chance to choose a song, even visitors!
-- Attendance isn't taken, but the social aspects are not neglected and singers often become good friends and absences are noticed. "Religion and politics get left at the door; we're here to sing" is a tradition upheld even at the monthly singings and this allows a quite disparate group of people to mingle and share a love of this music all without the tensions that might otherwise erupt.
-- There's no potluck meal, usually, and singers bring their own beverages/water bottles.
-- Many of the monthly singings are held at local churches because those tend to have better acoustics than private living rooms and are available. Many of them charge the singers a small fee to cover lights and heat, and so there is often a hat passed at the monthly singing to cover this fee.
-- Those lucky enough to live close to several monthly singings can attend them all and thus get to sing more than once a month. There's no rule says you have to attend only one, or every time.