When workers become unemployed due to a seasonal dip or reduction in production at a certain period of the year.
Below is a diagram to show the effects of this type of unemployment on the economy. The SRAS curve shifts inwards as temporarily workers become unavailable after periods of the year e.g. students have to return to education in September and therefore can only work full-time in the summer months. But this is only temporary and soon this effect gets reversed as workers make themselves available for the next seasonal time of the year e.g. Christmas time. So, in the long-run is there no change to the equilibrium as these effects roughly offset each other.
