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
April-1906 June-1906
July-1906 August-1906
September-1906
December-1906
THE CHARLES H. MOORE OIL CO.
January-1908
May-1908
July-1908
HOTEL HAVLIN
September-1914 March-1915
July-1916
August-1916
March-1917
July 1917
August-1917
PALACE HOTEL
THE STORRS-SCHAEFER CO. SPORTSMAN'S SHOT WORKS
August-1912
March-1914
March-1911 September-1911
THE R. A. KELLY CO. THE CALVERT ANILINE & CHEMICAL CO.
August-1912
May-1910
July-1910
October-1912 December-1912
D. H. BALDWIN & CO.
June-1906
August-1906
December-1906
January-1907
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
July-1916
Back of July Card
August-1916
LITTLEFORD SCHOOL
CAMPBELL COMMERCIAL SCHOOL
BARKER & FREISENS JEWELERS
CYPRESS LUMBER
FOR MORE CALENDAR
CARDS 
