You might call this Y Day on Metro. Almost all the bus lines that until now have crossed from Virginia into Washington will stop providing that service. Most of their passengers will shift to the subway's Yellow Line, which has been operating since April 30. Some may use the 6-year-old Blue Line.

Since there is only sketchy bus service on Sunday, the impact will really be felt for the first time during commuting hours tomorrow.

Preparing for the changeover, which many contend will provide worse and not better service for them, Metro has rejiggered many bus routes to feed into the Pentagon bus-rail transfer station and, to a lesser extent, the National Airport station. The list took most of a page in a recent Virginia Weekly section of The Post.

That list brought home once again the utter chaos of the Metrobus route-numbering and route-naming system. In a decade of bus operation, the transit authority has done little, if anything, to make sense out of this tangle. In fact, in integrating the four former private companies (and their five separate route-numbering systems), Metro has--if anything--made it incomprehensible.

In 1974, a year after the takeover of the private companies, Metro commissioned a transportation engineering consultant, Wilbur Smith & Associates, to try to bring order out of the chaos. Smith recommended that Metrobus go to a three-digit bus numbering system, as used in London.

The first digit would indicate the quadrant of the city or the suburbs the line operates through. The second and third digits would represent the route and its variations. Instead of a 7-A, 7-B and 7-C bus, it might be 701, 702 and 703. It makes sense.

Not wanting to upset longtime riders, and having major subway-financing problems to deal with, the Metro board shelved the whole idea.

The other day I asked Metro if and when it will return to that subject.

Beverly Silverberg, Metro's public affairs director, checked and reported back that Metro's staff is conducting its own study of bus route restructuring, bus numbering and destination signs. I won't hold my breath. But clearly it's a priority issue.