You can't do this with regular mail merge as you envisage. The easiest way to achieve it is to create a separate record for each month for each person i.e.

Jane Doe January 2015
Jane Doe February 2015
Jane Doe March 2015
Jane Doe April 2015
Jane Doe May 2015
Jane Doe June 2015
Jane Doe July 2015
Jane Doe August 2015
Jane Doe September 2015
Jane Doe October 2015
Jane Doe November 2015
Jane Doe December 2015
Bob Smith January 2015
Bob Smith February 2015
Bob Smith March 2015
Bob Smith April 2015
Bob Smith May 2015
Bob Smith June 2015


Bob Smith July 2015
Bob Smith August 2015
Bob Smith September 2015
Bob Smith October 2015
Bob Smith November 2015
Bob Smith December 2015
etc

Then it is a simple label merge.
Mailmerge with Word 2007/2010


If your label sheets are in multiples of 12 you can get away with inserting the name fiels alone, and fixed text for the dates, with a Next record field after every 12 records, but it is easier to create the worksheet with the data you require, given the power of Excel's copy down functions - especially as there are only 25 individuals.

The only other solution is to create the process in VBA to write values to the individual labels, but for that the label layout would need to be known. before you start as the variations are legion.

__________________
Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com