This study applied the modified spatial similarity coefficient method to define the seasonal transition (ST) from winter to summer in the extratropical stratosphere of the Northern Hemisphere. The features of the ST were examined using European Centre for Medium-Range Weather Forecasts (ECMWF) Interim reanalysis data; and the results showed that the time and duration of the ST, which is affected by the activity of planetary waves (PW) in the stratosphere, largely depended on the geophysical locations. This study also investigated the interannual variability of the ST and its relationship with stratospheric sudden warming (SSW) and the quasi-biennial oscillation (QBO). It was shown that the late-onset SSW events (after 22 January) are close to the start of the ST. An easterly (westerly) QBO hastens (delays) the onset of the ST in high and low latitudes, whereas it delays (hastens) the ST in midlatitudes. The duration of the ST is significantly affected by the QBO. The influence of SSW and the QBO have different significance in different latitudes, so they are both important and irreplaceable factors.