Seasonal affective disorder, also known as seasonal depression, causes depressive symptoms during only certain times of the year... and there's a pattern to it?!
In either case, at a 
certain time of the year,
In beiden Fällen fühlt man sich
zu einer bestimmten Jahreszeit
En ambos casos,
En ambos casos,
Dans les deux cas,
In ogni caso, in un determinato periodo dell'anno
Em ambos os casos, 
em certas épocas do ano,