A good web analytics tool will offer date range comparisons such as week to week or month to month.  One of the frustrating caveats with month to month comparison is that consecutive months have a different number of days.  One way of resolving this is to normalize the data in one of the months with a mathematical equation such as:

    (total page views) +/- (difference in days) * (total page views / days in month)

Let's assume that your web analytics vendor was savvy enough to provide date range comparisons, but doesn't do such normalization for you on the fly.  Assuming your report shows the percent differences between months, here's a simple way to normalize the data on the fly.  You may even stop using the fancy normalization feature provided by your web analytics vendor!

If the difference in days is one day (for example, March to April), then there is a 3.3% difference in days.  In other words, any percent difference over +/-3.3% is more likely to be a true difference month over month.  For a two day difference, it's just double (+/-6.6%).  And for a three day difference, it's triple (+/-9.9%).