So I would have January 2013 for instance in a stack to be folded, addressed and stuffed.

Hand folded looks fluffier.  

I know a company who does real folding and inserting...

:Just bought a machine to fold and insert 5k to 20k per month direct mailers.  

Easiest approach will probably be to create a formula in excel and then have the "xxxx" part concatenated in with the other parts and then write a VBA macro or use a software library to spit the resultant string to separate doc files or separate pages on the same doc file.Or you may be able to get Excel to print the information pretty enough so that all you would need to do is fold up the printed document or cut along the dotted lines.

@Dev Horn Here is a sample of the C fold we are enjoying currently.Inside click link to enlarge imageOutside and inside Click link to enlarge image

It took me 6 hours to fold, stuff, and stamp 200 envelopes. 

little 4x6.25, 6 x 9, 6 x 11 and 12 x 15Zip Letters - 2 fold configurations black only, full color and partial colorProfessional Letters OMG there are a ton of optionsBandit postcards - 2 sizes multiple colorsExpress Envelope Letters...

has anyone tried tri-folding their yellow letters and addressing the back?

To anyone who is a newbie and trying this method for the first time,  it will be like finding a needle in a haystack while blind folded?