Regression analysis for more than one categorical variable in time series

I have a time series data for shipment with following variables:

1. Year: 2008, 2009, 2010, 2011, 2012, 2013
2. Month: jan, feb, ..., dec
3. Number of ordering days
4. Shipment Volume

I want to know the effect of the number of ordering days on the shipment volume.

Can this can be done using regression method? If so, should I consider month and year as categorical variables?

• You might want to look at the US Census Bureau's X13 software census.gov/srd/www/x13as – david25272 Feb 5 '14 at 2:19
• You want to fit a time series model, which will take the correlation over months into account. – Brandon Sherman Mar 10 '16 at 5:01

Using different types of independent variable codings for (in the link's notation $\mathbf{x}$) in R is well described by this page