Americans are sending fewer holiday greeting cards in 1978 but more packages. Moreover, the telephone company expects a record number of calls during the coming weekend.

U.S. Postal Service officials said yesterday that letter mail to date in December is somewhat below the some period last year: 1.2 billion letters in 1978 compared with 1.3 billion in 1977.

Monday was the most active day in history for the nation's post offices, when they handled 145 million letters compared with 135 million on the comparable day in 1977. Tuesday volume also was ahead of the 1977 level but letter volume was off earlier in the month.

Including al types of mail, postal volume in December was 6.2 billion pieces through Tuesday, compared with 6.1 billion in 1977.

American Telephone & Telegraph Co., meantime, is expecting a record number of more than 17 million interstate calls-up 1.5 million or more from last year-in part because Christmas and Hanukah will be celebrated this year on the same day, next Monday.

Chesapeake & Potomac Telephone Co., the AT&T subsidiary that serves metropolitan Washington, has forecast more than 2 million customer calls placed by dialing and another 190,000 calls through operators on Monday.

"People call from home, instead of their places of business, and holiday conversations last about twice as long as business calls. This shift causes heavy traffic on usually low-volume routes," said C&P's spokesman, Web Chamberlin.

As a consequence, many C&P workers won't have a holiday. Chamberlin suggested that customers place calls on Sunday or between 1 p.m. and 5 p.m. on Christmas/Hanukuh day - the projected slackest calling period. A lower holiday rate will be in effect from 8 a.m. to 11 p.m., even though the holiday falls on a normal workday.