Within 20 days this month, as summer temperatures soared, five Fairfax County residents were slain in separate cases that have members of the county homicide squad working around the clock and on their days off.

It was a remarkable contrast with the trend of recent years. The number of murders in the county had plummeted in a four-year period, from 52 committed in 1976 when seven persons died in one mass slaying, to a low of only 11 last year.

"It's one of the oddest things about murders," said Fairfax County's chief prosecutor, Robert F. Horan. "They seem to run in packs. I remember a period in '73 and '74 when there were none from February to September. Then we had seven in a six-week period.

Both Horan and Maj. James Joseph of the Fairfax Criminal Investigation Bureau said a rash of murders is not unusual and does not indicate a crime wave.

Horan added that he doesn't "think there is much doubt that more murders occur in the hot months, in the summer and early fall."

Susan Mull, criminal justice planner for the Northern Virginia Planning Commission, disagreed, saying there is no known way to attribute spurts of murderous activity to outside factors.

"There is no known correlation between violent criminal acts and economic conditions, employment, or other situations," Mull said.

There were no homicides in Fairfax City in 1979, and only one in 1978. There have been three thus far in 1980, according to Fairfax City police. Arlington murders are up to four this year, the same number that occurred in 178 and 1979 combined.

"We're having a 300 percent increase," a Fairfax City official remarked wryly. "The first one in January was drug related and the second occurred in a fight outside a restaurant. Tensions are up."

The upswing in murders in the Virginia suburbs contrasts with the homicide rates in Maryland and the District of Columbia. Washington police report they have recorded 101 murders this year, down from 132 at the same time last year.

Prince George's County detectives said they have had 24 homicides so far compared with 42 at this time in 1979. In Montgomery County the number of slayings recorded so far this year is nine, three fewer than at the midpoint of 1979. Four of these nine, however, occurred from July 20 to July 27, imposing a heavy burden on the county's detective force.

The summer has not brought an upsurge in murders in the District. D.C. police said yesterday they have had 23 slayings in June and July compared with 37 in the same two months a year ago.

As in most other jurisdictions, Fairfax County officials said, many of the murders involved persons who were well acquainted with each other, and were not random slayings. Once that is stated, similarities in the crimes are few, police say.

"In some cases, arrests are made within 24 hours, or we at least have a prime suspect," said police spokesman Warren Carmichael."If it doesn't happen that quickly it will generally take some time to solve."

Although certain cases that have reached dead-ends may be classified as "inactive" or "dormant" Fairfax's Mayor Joseph said most detectives "get emotionally involved in the cases. They aren't closed until you've made an arrest.

"You owe the victims something. So you give it overtime, your free time. The case becomes a part of you," Joseph added.

This year the Fairfax detectives say they have managed to clear all but two of the county's 10 murders.

Taken as a whole, Northern Virginia homicides for the jurisdictions of Fairfax, Arlington and Prince William Counties, and Alexandria, have dropped steadily from a high of 88 in 1976 to only 25 in 1979, according to state statistics.