Upperville Colt, Horse Show Starts Tuesday at Grafton

The 137th annual Upperville Colt and Horse Show begins Tuesday and continues through June 10 at Grafton Farm on Route 50 between Middleburg and Upperville. The show opens each day at 8 a.m. and continues until dusk. Jumping competitions are scheduled for Wednesday and June 7, 9 and 10. Admission is $3 during the week and $5 on the weekend. Children younger than 12 are admitted free. Call 347-5863 before the event and 592-3858 during the show. Seminar on Real Estate Planned

The Loudoun County Chamber of Commerce will offer a seminar on real estate assessments from 2 to 5 p.m. Saturday at Simpson Middle School in Leesburg. Speakers include Virginia state Sens. Charles L. Waddell (D-Loudoun) and Virgil H. Goode Jr. (D-Franklin), Del. David G. Brickley (D-Woodbridge) and County Assessor William Gardner. Admission is free. Art, Antiques Needed for Auction

The Waterford Foundation needs contributions for its art and antiques auction on June 24. The foundation, established to protect the historic landmark status of the village of Waterford and its surrounding area, is seeking paintings and art objects, furniture, rugs, old books and services such as landscaping, dinners for two and tickets to athletic events or concerts. Call 882-3018 or 478-8454. Workshop to Keep Clocks Ticking

The Douglass Community Center in Leesburg will sponsor a workshop on care and maintenance of clocks, with an emphasis on grandfather clocks, at 10 a.m. Saturday. Admission is free. Amateur Night Continues

Home Place Open Mike, an evening of amateur entertainment at the Lucketts Community Center, continues at 8 p.m. tomorrow. Admission is $1 and includes refreshments. Call 777-8881 if you want to perform. Fishing Clinic at Sky Meadows Park

This weekend at Sky Meadows State Park: a fishing clinic from 2 to 4 p.m. Saturday and a mock Confederate encampment and display from noon to 4 p.m. Sunday. Bring rod, reel and tackle to the fishing clinic. Events are free but there is a $1.50 parking fee. Call 592-3556. NOVA Sets Summer Registration

The Loudoun campus of Northern Virginia Community College is holding registration for summer classes. Registration for the eight-week term from June 14 to Aug. 8 is open until June 12. Registration for the six-week term from June 28 to Aug. 8 is open until June 26. Call 450-2501. Loudoun Youthfest at Courthouse

Youthfest, Loudoun's third annual cultural festival for high school students, will take place from noon to 5 p.m. Saturday on the courthouse lawn in Leesburg. Entertainment will include musicians, singers, drama and forensic demonstrations, and exhibits from school clubs and literary publications. Representatives from county agencies will be on hand to answer questions about county youth programs. Call 777-0358. Piano Concert at St. James Church

Faculty members of the Shenandoah College and Conservatory of Music will present a piano concert featuring 18th century music at 7:30 p.m. Saturday at St. James Episcopal Church, 14 W. Cornwall St., Leesburg. A $5 donation is requested. Call 777-1124. Day Camp for Asthmatic Children

The American Lung Association of Northern Virginia will sponsor a day camp for children with asthma and their families from 9 a.m. to 4 p.m. June 16. The deadline for registration is Monday. Call (703) 591-4131.