THE JAPANESE had been counting on celebrating their 1977 National Day on April 29 in their new embassy complex on Nebraska Avenue, but Washington's severe winter ruled this out.

Japanese National Day will be observed, as in the past, at the present embassy residence on Massachusetts Avenue at California Street.

Japanese Ambassador and Mrs. Fumihiko togo believe they will be able to move to the new embassy residence in September.

Many Japanese cherry trees have been planted on the extensive grounds, a new fountain is in place and the new residence is ready and waiting.

"But, mostly, the grounds are a sea of mud right now," says an aide.

Weather and zoning have held up plans of other embassies to build as of now. The French, who have acquired an 8-acre tract of meadowland on Reservoir Road across from Georgetown University Hospital, have been delayed by zoning problems. It looks now as if they will not break ground until 1980. Their acreage is a part of the Hillandale estate of John D. Archbold, acquired in 1973. Archbold still owns 32.2 acres of the estate.

There have been many conferences with neighbors about parking and traffic. But the French feel that an underground garage for 600 cars will take care of that.

Of course, the French are not giving up their present elegant embassy residence on Kalorama Road.

There will be four main buildings on the new site. The first will house the chancery or diplomatic offices now scattered around town. The second will contain cultural and scientific services. The third will have reception, exhibition and lecture halls. And the fourth building will house commercial and financial services. In addition, the grounds will contain a recreation area plus tennis courts and swimming pool.

The Italians, who have acquired the 22-acre estate of the John A. Logans, Firenze, are doing some minor redecorating to the main house. Ambassador and Mrs. roberto Gaja hope to move in within the next couple of months.

But plans for a new chancery complex and other buildings will not get under way for at least another year. The present embassy residence on 16th Street will be used to expand the chancery after the Gajas have moved out and into Firenze manor house.

The Embassy of Ghana has space reserved in a new diplomatic complex being built on connecticut Avenue at Van Ness Street. But building has been delayed so far, and no one expects for anything to get started for six months.

Ambassador and Mrs. Rolf Sieber, of the German Democratic Republic, moved into their new diplomatic home six months ago. but building and redecoration there are still going on.

The East German Republic purchased the luxurious modern home in Chevy Chase, which was once the site of the historic Rossdhu Castle. The house was bought from Mr. and Mrs. Herbert H. Haft (he is president of Dart Drugs).The handsome house in its woodland setting has five bedrooms and 9 1/2 baths. There is a swimming pool and a 40-feet long playroom, designed for dancing with an orchestra. Rossdhu Castle, originally on the site, was a copy of a Scottish castle on Loch Lomond and was torn down in 1957 to make way for the Haft's modern home.