THOSE OF YOU who enjoyed a quiet holiday at home last Wednesday may be excused from reading this--because you were spared the awful experience of trying to use this region's mass transit system on a ''half-holiday" to get to school, work or Christmas shopping. Metro blew it, by design, with a schedule that didn't begin to serve the day's ridership on time. If its would-be customers were moved, it was probably to tears or to sound off.

Yes, Metro does explain its holiday procedures-- that is, if you happen to spot them in an overhead advertisement on your bus, or maybe in a little holiday schedule appearing in the newspaper. But let's say you didn't know on Wednesday that Metro lists Veterans Day as being on its "Saturday schedule"-- unlike, say, Christmas Day, which is a "Sunday schedule" day. The difference between these two schedules on some routes is considerable: between skeleton service and no service at all.

Tried to call? Then you know--none of the Metrominds thought to slap an advisory message on the phone-tape telling you that Wednesday would be a Saturday; and if you waited long enough for a live response, you missed the holiday anyway.

What if you already did know that buses would be on a Saturday schedule? You waited--and then watched as that one and only bus blew on by, filled to the gills and followed by nothing like it for the next 20 minutes to half an hour. According to our totally unscientific spot survey, the mess was day- long at many stops.

Yes, it is not easy to predict how many people will work, not work, shop or not shop on Veterans Day, Columbus Day, George Washington's Birthday or any other holiday-like event. But Metro's Saturday schedule isn't much to ride home about in any case --and officials should think seriously about beefing it up when past patterns point to such a need. Certainly all veterans of the day last Wednesday can attest that it was a case in point.