Regardless of the sign of the zodiac under which you were born, this could be a dangerous day for you.

It could be an especially dangerous day if you have recently been weighing the advisability of kidnaping somebody for ransom.

And it can be even more dangerous if you have read about the arrest of John J. O'Grady in California on a charge of kidnaping.

Staff writers Ronald D. White and Joe Pichirallo informed us yesterday that FBI agents knew exactly where to look for O'Grady. they had about as much trouble following O'Grady's trail to a San Francisco hotel room as they might have had in tracking an elephant through six feet of snow.

The search for O'Grady began shortly after 19-year-old Carol Lynn Drewer was kidnaped in McLean last week. the young woman is the daughter of bank president Milton Lee Drewer Jr.

A ransom of $50,000 was demanded. The banker paid it. The girl was released.

Thereupon, says the FBI, the kidnaper began making one foolish mistake after another. Suspicion soon fell upon O'Grady.

One agent called the suspect's actions "unbelievably amateurish." Personally, I do not find it unbelievable when a kidnaper's actions are amateurish; after all, very few people ever get a chance to become professionals at kidnaping. Pressure from lawmen doesn't permit kidnapers much time for on-the-job training.

Despite what the FBI says, O'Grady will be presumed innocent in this column until a jury finds otherwise. However, many who read news accounts of cases of this kind form opinions of their own -- and one of those opinions is, "Boy, this guy was really stupid. I'll bet I could have pulled it off without making all those mistakes."

Thinking that you can avoid all the mistakes and pull off a perfect crime, or at least a crime that lawmen won't solve, may be the biggest mistake of all. It tempts otherwise rational people to take a poor gamble. Look at the odds: There is only one of you but uncounted thousands of them. And collectively they know more tricks and stratagems than any single criminal ever will. They have computers, communications networks, firearms, scientists, time, money and the full power of the state behind them. Yet every time we publish a story about a crime that was easily solved because the perpetrator made foolish mistakes, there are some who think, "I'll bet I could have pulled it off."

Don't kid yourself. It is extremely difficult to plan an illegal act that leaves no evidence in its wake, or to flee without leaving a trail, or to assume a new identity without risk of mishap.

And even if you can get over all those hurdles, the last one will trip you up anyhow: When the cops want you badly enough, they'll probably find you.

Then you'll have a long time to ponder your mistakes and to talk yourself into believing that if you ever get out of jail you'll know how to avoid all the mistakes and pull off a perfect crime.

The first mistake to avoid is the notion that you can beat the system. Our prisons are filled to overflowing with people who didn't think those dumb cops would ever catch them. BARGAIN DAY

Russell C. Lynch of Arlington received an interesting communication from North American Minerals last week. It was loaded with exciting words that indicated Lady Luck had smiled upon Lynch. The message was:

"Your name was selected and computer-printed on this card as part of an international publicity campaign conducted in cooperation with Canada's largest refinery of precious metals and supplier of silver to the Canadian mint.

"If you are able to mail the attached claim form before midnight Oct. 31, 1980, we will ship a solid ingot of pure silver from our vault to your address for the sum of $10 plus $2 for transport and insurance.

"Your five troy gram ingot is a solid bar of pure silver of .999 fineness (the highest grade of silver)."

There is "a limit of one ingot per address," but those who respond before Oct. 23 can buy two. Also two neck chains at $10 each.

Lynch does not intend to buy any ingots or chains.

He says, "my calculations show there to be 31 grams in a troy ounce, and with silver at about $24 an ounce, a gram would be worth 77.5 cents, and five grams would be about $3.88."

Good heavens, Russ, aren't you impressed to know the Canadian mint buys some of its silver from the same source? Or that a computer selected your name? Or that a claim form and a deadline are involved? Or that the ingot is solid and pure and is kept in a vault ? I guess you just don't have any romance in your soul, Russ -- or any interest in buying silver at almost $75 an ounce.