The nation's major retailers rang up widely mixed sales during the crucial Christmas season, which was six days shorter this year, but sales on the last three shopping days were exceptionally strong, analysts said yesterday.

The Christmas period is the most important for retailers, accounting for more than half of their revenue for the year.

For the major retailers overall, sales will be up 4 percent to 5 percent over 1984, or an average performance for Christmas, said Tom Tashjian, a retail analyst with the investment firm Prudential-Bache Securities Inc.

"An average Christmas is very acceptable," he said.

December sales at Wal-Mart, the nation's seventh-largest retailer, will be up more than 30 percent over a year ago, a company spokesman said. Sales at stores open more than a year will be up about a third of that.

Sears, Roebuck & Co., the largest retailer in the country, however, was unsure it would report any kind of gain over last Christmas.

K mart Corp., the No. 2 retail chain, has seen a drop in sales at stores open more than a year, while No. 3 J. C. Penney Co. may report a slight increase, analysts said.

Unseasonably warm weather discouraged shopping early in December, and sales were stronger on the coasts than in the rest of the country, where manufacturing and farming have been hit hard, analysts said.