The Fourth of July weekend may signal the height of summer to some, but for the American Red Cross it marks the beginning of a two-month drain on the area's blood supply.

More accidents during holiday weekends usually increase local hospital demand for blood by about 10 percent, according to a spokesman for the regional office of the Red Cross.

In addition to the problems of increased demand, blood collection also drops during the summer because many people are away on vacation. The local Red Cross collection of 850 units of blood a day drops to about 600 units in the summer. Consequently, "by Labor Day weekend, we have to go on an emergency blood appeal," said spokeswoman Carbery Morrow.

The local Red Cross provides 90 percent of the blood used at 60 hospitals in the District, Maryland, Virginia and West Virginia, Morrow said.

Trying to head off a critical blood shortage in coming weeks, the Red Cross is asking area residents to donate blood now at centers around the region.

Type O blood is universal and the most common type, Morrow said, adding that Type B blood, which is more common among blacks, is in high demand in the District.

Three blood donation stations will be open this weekend:

Georgetown University Village C at 37th and O streets NW in the formal lounge from 10 a.m. to 3 p.m. tomorrow.

Prince George's County Red Cross Chapter, 6206 Belcrest Rd., Hyattsville, 9 a.m. to 1 p.m. tomorrow.

St. Matthews Methodist Church, 14900 Annapolis Rd., Bowie, 9 a.m. to 2 p.m. Saturday.

Further information about Red Cross schedules and blood collection sites is available by calling 728-6550. Several hospitals operate their own blood banks and they too are seeking donations.