Just how grim was the Christmas season for the nation’s retailers? The bad news comes today — at least officially — when major chain stores report December sales figures.

If preliminary estimates are correct, the holiday season will go down as one of the worst in more than 40 years.

But with little reason for consumers to spend in the post-holiday period and an excess of clearance inventory, January could deliver yet another retail blow.

Nearly every retail category is expected to post significant same-store sales decreases for December compared with a year earlier, according to Thomson Reuters estimates.

Wal-Mart Stores Inc. probably will be one of the only stores to do well. The discount giant is expected to record a 2.8 percent sales increase in December compared with the year-earlier period.