Throughout the Washington metropolitan area, most of the activities of Halloween will be held this year on Monday Oct. 31, the traditional date for the holiday.

Last year, anxious trick-or-treaters and their parents were thrown into some confusion and disarray when Oct. 31 fell on a Sunday. It was not clear whether celebrations would go forward, according to custom, or whether an alternate day would be adopted as "Halloween."

The Gaithersburg City Council was the only local government body to step forward and proclaim Saturday, Oct. 30, as Holloween a year ago. Other jurisdictions left the choice up to the boys and ghouls.

This year, with Oct. 31 coming on a Monday, the way is clear to continue Halloween business as usual.

Police departments in Montgomery, Prince Georges and Fairfax counties and the City of Alexandria said they have arranged to put additional patrols on the streets Monday night to guard againt vandalism and other problems. Arlington County and District of Columbia police said no extra patrols will be assigned, but officers will be on the lookout for unusual problems throughout the evening.

A spokesman for People Drug Stores, which operates 129 outlets in metropolitan Washington; said that costume and candy sales dropped slightly last year, apparently because of the mix-up over just when Halloween would be celebrated. This year's sales seem slightly above normal, the spokesman said.

Among the dozens of Halloween gatherings that will be held during this season, many will be fund-raisers for UNICEF, the United Nations Childrens Fund.

Irene Davidson, manager of the UNICEF Information Service in Washington, said that although more children and adults participate in organization's benefits's each year, fewer UNICEF participants are going from door to door as trick-or-treaters.

"People seem to feel safer and a little more comfortable if their children are going to an activity that's indoors, perhaps with classmates at shcool or at a church," said Mrs. Davidson.

An All Hallows Celebration will be held at noon Sunday at the Congressional Cemetery, 18th and E. Streets SE. The event, to be highlighted by clowns, box lunches and a performance by the Folger Shakespeare theater group, is designed to raise funds for the Association for the Preservation of Historic Congressional Cemetery.

At 7 p.m. Sunday at Market Square in downtown Alexandria, the Jaycees and city department of recreation are jointly sponsoring a costume contest, witch's walk and live snake show.

Today in Prince Georges County at Greenbelt Park, the National Park Service is sponsoring a halloween Festival, complete with exercises in pumpkin-carving, a costume contest and apple-bobbing.