I would start out with 2-way ANOVA (Analysis of Variance), using the variables:

(1) Month/Year

(2) Sample Number

ANOVA will tell you if the sample-to-sample variation, within each month, is significant, while controlling for month-to-month variation. It will also tell you if there is an interaction between months and samples.

Do a means plot for each variable, and also an interaction plot - try to look for patterns (is sample 1 always bigger than sample 2? is one sample bigger in particular months every year? are there other interesting patterns?)

A good place to look for info is here:

http://davidmlane.com/hyperstat/factorial_ANOVA.html