This tutorial talks about how to do seasonal decomposition by Loess.
The Seasonal Trend Decomposition using Loess (STL) is an algorithm that was developed to help to divide up a time series into three components namely: the trend, seasonality and remainder. The methodology was presented by Robert Cleveland, William Cleveland, Jean McRae and Irma Terpenning in the Journal of Official Statistics in 1990. This paper can be downloaded here. Read more R in Time Series: Seasonal Decomposition By Loess