The latest cut in the discount rate helped propel the stock market to a gain today, although this turned out to be one of the slowest trading sessions of the year.

A quiet day had been expected with many investors taking a long Christmas weekend. The markets will be closed for the holiday on Tuesday.

Analysts cautioned that the low volume made it difficult to draw firm conclusions about the day's trading.

Phillips Petroleum fell sharply in today's trading as a result of an agreement that ended a takeover threat from a group led by T. Boone Pickens Jr. Gains were numerous among other oil issues and technology stocks.

The Dow Jones industrial average gained 11.16 points to 1,210.14.

With four trading days remaining in 1984, the Dow Jones industrial average shows a net loss of 48.50 points since last New Year's Day.

The New York Stock Exchange index jumped 0.62 point to 96.18 and the price of an average share increased 21 cents. Standard & Poor's 500-stock index gained 1.25 points to 166.76. Advances topped declines by 970 to 457 among the 1,919 issues traded.

Big Board volume totaled 55.55 million shares, down from the 101.27 million traded Friday. The Christmas Eve pace was the second slowest of the year, after the 46.36 million shares traded on Oct. 8.

The cut in the discount rate from 8 1/2 percent to 8 percent announced by the Federal Reserve after the stock market closed Friday encouraged investors. The discount rate last was cut on Nov. 21, sparking a rise of 18.78 points in the Dow Jones industrial average in the next trading session.

Many analysts expect the reduction in the discount rate to be followed by cuts in the prime lending rate, perhaps to 10 1/4 percent or even 10 percent.

John Burnett of Donaldson, Lufkin & Jenrette said the advance had to be viewed cautiously because the volume was so light. He said a real test for the market will come later in the week.

John Smith of Fahnestock & Co. said the extent of the advance was somewhat surprising because many institutional investors are low on cash. He said the stock market should do better in 1985 because most people feel the economy has turned around.

"We think this half-point cut in the discount rate was already in the price of stocks," said William LeFevre of Purcell, Graham & Co. He said a reduction of a full point would have spurred stocks sharply higher.

On the other hand, he said that "lower interest rates are good for bonds immediately and stocks eventually . . . the Fed recognizes the economy needs some stimulation and is providing it."

"We'll see more of a two-sided market Wednesday and Thursday as we get players back," said Larry Wachtel of Prudential Bache. He said the discount rate cut was "not exactly a surprise." He said seasonal factors might help the stock market the rest of the week, plus the expiration of stock index options over the weekend.

Composite volume of NYSE issues listed on all U.S. exchanges and over the counter totaled 63.74 million shares, down from 121.53 million on Friday.

The American Stock Exchange index gained 0.91 point to 202.41. The price of an average share increased 5 cents. Advances topped declines by 345 to 168 among the 765 issues traded. Composite volume totaled 5.92 million shares, down from 7.99 million on Friday.

The National Association of Securities Dealers index of OTC stocks jumped 1.54 points to 245.82.

It was a less-than-merry Christmas Eve for investors who had bought Phillips Petroleum stock on news of a planned takeover of the company by a group of investors led by T. Boone Pickens Jr., chairman of Mesa Petroleum Co.

Over the weekend Pickens' group and Phillips reached an agreement to end the bid.

Although the agreement was set up with measures designed to cushion the impact on Phillips' stock price, the shares traded today at 45 1/4, down 9 5/8 from Friday. More than 4.8 million Phillips shares changed hands, putting the issue at the top of the active list.

Mesa dropped 5/8 to 18 1/8, also in active trading.

Some other energy issues rose on speculation that they might become Pickens' next target. Mobil gained 1 5/8 to 28 3/4. Exxon 1/2 to 44 3/8, Sun Co. 1 7/8 to 47 1/4, Unocal 1 1/8 to 36 3/8, Texaco 5/8 to 34 1/8, Atlantic Richfield 3/8 to 43 5/8 and Chevron 7/8 to 31.

Scovill climbed 1 to 39 1/2 on top of a 9 1/8-point rise last week, when members of the Belzberg family of Canada proposed a $35-a-share takeover of the company. Scovill opposed the offer amid conjecture on Wall Street that a higher bid might be forthcoming from another party.

AT&T (ex-dividend) was second on the active list, gaining 3/8 to 19 1/2.

Commonwealth Edison (ex-dividend) was third among the actives, adding 1 1/8 to 27 1/8.

General Motors added 3/4 to 78, and Ford went up by 3/8 to 45 1/4.

Union Carbide jumped 1 to 38.

In the technology group, IBM gained 1/2 to 124 1/4, Digital Equipment 1 3/4 to 111, Texas Instruments 1 3/8 to 117 3/8, Advanced Micro Devices 3/4 to 29 3/4, Motorola 1 3/8 to 35 1/8, Data General 1 1/2 to 59, Burroughs 1 1/8 to 58 1/4 and National Semiconductor 1 3/8 to 11 7/8.

Commodore International, which fell 2 Friday, was off 1 1/2 to 16 3/4. The company said computer sales in North America were lower than anticipated.

On the Amex, Wang Laboratories class B was the most active issue, up 1/4 to 25. TIE Communications was second, unchanged at 6 7/8. B.A.T. Industries was third, up 1/16 to 4 1/4.