This is the one weekend of the year when the welcome mat is out for the public at several of the finest thoroughbred-breeding farms in Northern Virginia. The fee is fair, the cause is a good one and the weather forecast is excellent for Saturday and Sunday. Give your family a break. Saddle up the station wagon and go.

The cost is $8 per person a day or $17 for a husband and wife and all children under 16. One day suffices. If too many people were to return Sunday, those back roads would be jammed. As it is, more than 3,000 persons are expected to show up for the Hunt Country Stable Tour this time around.

Trinity Episcopal Church in Upper-ville benefits from the tour. Tickets may be purchased there, at Inter-House Farm near Middleburg or, on Saturday, at the Middleburg Training Center.

For anyone primarily interested in horse racing, as opposed to viewing a sumptuous breeding farm and estates, Saturday offers the better program inasmuch as there is an opportunity to observe the early morning activity at the training center.

At 8 a.m., the tour goes directly to the training track where breakfast is available in the track kitchen for trainers and riders. Tour members are invited to stroll through the barn area and to watch horses work over the training strip. Don't be afraid to ask questions. Except, maybe, how a particular horse is likely to run next week at Pimlico or Bowie.

Also open on Saturday only will be Foxlease Farm, an internationally famed dairy operation that has sold cattle-breeding stock to 17 countries on four continents. On display will be Foxlease Perseus Argus, an outstanding bull that has brothers in Italy and France.

Brochures that include complete maps of the tour will be distributed at various points. The intinerary is what you make it, with Mr. and Mrs. James Wofford's stable, Oakley Farm, Rokeby Farm, Blue Ridge Farm, Hickory Tree Farm, Waterford Farm and Inter-Horse all open for inspection.

Wofford, an Olympic medal winner, trains three-day event horses and develops aspiring riders at his new stable.

Oa'kley is owned by Mrs. A.C. Randolp, a lady who once helped a lad named Billy Turner get his start as a trainer. Turner has been in the news recently as the conditioner of Seattle Slew.

Blue Ridge is home for Cyane, Mongo, Salem, Farewell Party and On The Warpath. It is one of the busiest stallion stations in the East.

Inter-Horse will play host to a cocktail buffet at 5 each afternoon.

Before then, be sure to take a prolonged peek at Rokeby, Waterford and Hickory Tree, three of the finest farm operations anywhere.

Mill Reef, Arts And Letters and Fort Marcy were raised at Rokeby, which has one of the classiest broodmare bands ever assembled. Even Rokeby's rejects bring formidable prices at the occasional reduction sales.

Waterford is a new farm carved out of the countryside by William Haggin Perry, Lamb Chop, Alanesian and, last year, Revidere, have gained national championships for Perry, who has enjoyed exceptional success with the fillies he has raced.

Hickory Tree stands Favorecidian, Hagley. Limit To Reason, Salerno and Mystic II at stud and, like several other stops on the tour; it has its own training track where its young stock is broken and receives early training.

Racing fans will recognize the name of many of the horses to be seen on the farm. What's fun is finding out the breeding of a particular foal in one of the fields and filling away that information for the future, to see how - and if - that foal develops as a future race horse.

It is not necessary, however, to be a $2 better to enjoy this tour. The countryside is gorgeous, the farms are beautifully appointed and the bloodstock standing in the fields and in the paddocks is of the highest order. just to see a mare with foal alongside at this time of year, in such a setting, is a sight to remember.

It's a grand way to spend a day off the beaten track. Enjoy.