With Google Analytics you can view any snapshot of your web traffic for any period of time. You can compare year over year, month over month and other time-ranges that are relevant to your business to help gain better insight into how your web visitors interact with your website.
By default, when you first open your Google Analytics account, it will always show you the previous 30 days. If you don't change the date range, and are clicking through to look at behaviors, conversions, social media interactions, and so on, it will show you the previous 30 day period of time.
You can change the date range to view any period of time you'd like. Of course, the data will show a flat-line with a value of "0" if you select a date that occurs before your Google Analytics tracking code was installed on your website. But you can select any date range up to today's date.
On every screen in your Google Analytics account you have the ability to change the date view (shown below) by either typing in the dates, or selecting from the calendar that expands for you.
If you want to compare web traffic and interactions to a previous period of time, you can use the "Compare to" option by checking the box just below the date and by default it will add the dates for the range of time right before your date range. So if I chose a 15 day period in my date range, it would by default choose the 15 days prior to that. So Date Range = July 15 - July 30, Compare to dates would default to July 1 - 15.
You can use this feature to look at year over year traffic and look for trends and cycles as they may relate to your business's buying cycle and marketing campaign schedule. To do this you would set your date range to Jan 1, 2015 - Dec 31, 2015, and then compare to Jan 1, 2014 - Dec 31, 2014. (Shown below) The blue line is your current date range and the orange line is the previous or "compared to" date range.
And then there are 3 levels of granularity for your date ranges. You can view them as daily stats, weekly stats, or monthly stats. This is done by click the toggle buttons in the upper right corner (shown below) on every screen just below the date range selection box. In the example below you can quickly identify in the view by month that in May-June time frame traffic was up this year over last. You can then look at what you did in terms of marketing, or events, or even what external situations impacted your web traffic or sales.
If you are party rental business in the north eastern United States, you will likely see traffic spikes from May through October, and then a traffic lull. An online retail site might see spike around holidays or email campaigns. Where a company that sells portable generators may notice a huge spike in traffic that doesn't align with any campaigns. Looking at that period of time you might identify that there was a major storm that left locals without power and caused a spike in traffic and sales - this is what I would define as an external influence.
It's always good to do a year-over-year comparison.