Calendar cards are just what the name implies, cards that were sent out from many companies and institutions advertising their products and services. They were sent to potential customers each month, and was probably a costly enterprise for them back then. Many of them had holes pre-punched so they could be hung on a nail or hook. I don't know if anybody specializes in these cards but if they do it must be almost impossible to get a complete set of 12, and who knows how many years they put these out.
BRADFORD BELTING
March-1906
July-1906
September-1906
December-1906
January-1907
THE CHARLES H. MOORE OIL CO.
May-1908
July-1908
HOTEL HAVLIN
September-1914
July-1916
August-1916
March-1917
July 1917
August-1917
PALACE HOTEL
February-1911
THE STORRS-SCHAEFER CO. SPORTSMAN'S SHOT WORKS
March-1914
March-1911
THE R. A. KELLY CO. THE CALVERT ANILINE & CHEMICAL CO.
August-1912
May-1910
July-1910
D. H. BALDWIN & CO.
June-1906
August-1906
December-1906
January-1907
July-1905
THE CHATFIELD & WOODS CO. THE MARMET COAL CO.
Same
picture-same month-different companies.
March-1907
April-1906
THE TRIUMPH ELECTRIC C0.
WHITE RIVER CREAMERY CO.
February-1916
March-1916
August-1916
LITTLEFORD SCHOOL
June-1911
July-1911
September-1911
January-1912
March-1912
CAMPBELL COMMERCIAL SCHOOL
April-1912
BARKER & FREISENS JEWELERS
December-1912
January-1913
FOR MORE CALENDAR
CARDS 
