In compiling statistical tables results are often distorted by variations which are directly attributable to seasonal influences. Retail sales figures will tend to show substantial increases in the month prior to Christmas; employment figures will also tend to rise at that time, as they will at other periods of peak demand, e.g. in the tourist industry during the summer. These seasonal peaks, or troughs, are not representative of real trends, and in providing fair and comparative figures statistical techniques are used to eliminate the effect of these seasonal variations.
Reference: The Penguin Business Dictionary, 3rd edt.