This long weekend's holiday won't be an old-fashioned Fourth of July, with business at a halt and families at picnics.

The real birthday of the nation falls on Saturday, but because the federal government and many corporations have declared a holiday for Friday, normal business and working schedules have been discarded and a substantial part of the work force will be on the job both days.

The only sector of business that will be closed completely on Friday and Saturday is the investment community. All stock and commodities exchanges will shut down Thursday night and reopen next Monday.

Among financial institutions, however, some banks and savings and loan associations will be open on Friday, some will be open on Saturday, and some will close both days. There is no precise pattern, and customers should check with individual institutions.

In the District, as of yesterday, all commercial banks and most S&Ls planned to be open on July 3, but at least two S&Ls plan to close. The D.C. City Council passed emergency legislation yesterday making Friday a legal banking holiday, but one bank officer said it would take an act of Congress to allow city banks to shut down since they all have federal charters.

Generally, most Virginia institutions will be closed Friday, and most Maryland banking institutions will be open. Some suburban banks with normal Saturday hours plan to be closed this week. Some will be open. There is no standard for money-center banks, either. In New York, for example, most banks will be open Friday, and foreign-exchange traders will be in business.

The U.S. Postal Service has given responsibility for holiday-weekend operating decisions to regional authorities. Thus, there are some differences across the country. In the Washington area, the Postal Service has scheduled regular deliveries and pickups on Friday as well as regular window service at all but about 14 post offices that are located inside federal office centers, which will be closed.

Postal workers here will observe the holiday on Saturday, when all postal windows will be closed, and only express or special delivery mail will be sent out.

For most retail stores and workers, there will be no holiday at all. On Friday, area retail stores are planning to be open for normal hours. On the real holiday, Saturday, most retailers plan to be open from about 10 a.m. to 6 p.m.

Many corporations plan to close their administrative offices Friday for the holiday, but some plants have scheduled normal work hours.