A common phrase is to say something is the “Best thing since sliced bread”. This begs the question of what was the best thing before sliced bread. In order to answer this question, one needs to know - when did sliced bread get invented? I know that as soons as knives were invented, that bread was sliced, but at what point were you able to take home bread that was presliced from your bakery? Thanks
Otto Rohwedder’s automatic bread slicing machine was first used commercially in 1928.
Upon seeing the machine for the first time, people were heard to exclaim that it was “slicker than snot on a doorknob” 
ralphminer, welcome to the SDMB.
To answer your question, sliced bread dates to 1928, at least in Canada. Otto Rehwedder began work on a bread slicer in 1912, but sliced bread wasn’t marketed commercially until 1928. I got this from this website.
According to this site, however, sliced bread dates from the mid 20th century, which is in keeping with what I’d already read on the subject. It is possible that sliced bread was marketed in Canada before the US, or that in the US it was marketed regionally before it became available nationwide.
That said, if you do run across a loaf of unsliced bread, many bakeries are happy to slice it for you.
Robin
More about SB from previous threads:
http://boards.straightdope.com/sdmb/showthread.php?threadid=53423