Hypothesis testing

The Kruskal-Wallis H test is a nonparametric test for deciding if samples come from the same population. While the variances may not be equal between the two or more samples, they should not be significantly different, which would indicate that the samples are from different populations.
This may also be accomplished using a t test or the Mann-Whitney U test

Hello Paul,
You can do an ANOVA (analysis of variance) using Excel or any statistics package. Set up your data in columns. The ANOVA procedure is in the Data Analysis toolpak.
You can also compare two means using the z test or t test. That would be the simple way. A p-value less than .05 is significant.
Good luck.

Paul,
Don’t worry about the normality of your data. You are testing the means. Do you have any reason to believe one month’s distribution is a different shape than the next month? I don’t think so. Thus, the shape and spread of the distributions is irrelevant. And, after all, you are just checking at this point. If something useful appears you should do further investigation.  
I can’t say that you should totally ignore the normailty assumption  What I am trying to say is; you will probably get the information you need without that bit of worry. In economics, we would never base a theory just on a regression analysis. There must be some logic or reasoning behind it. Conversely, if there is no theory to support the idea that the distributions are of different shapes,  relax and test the hypothesis.
By the way, where are you? We are having this discussion in the middle of the night in the US. For me it is easy, I am in China.
Good luck.