The most characteristic diagnostic feature of SAD is the occurrence of symptoms of depression in fall and winters only. In this mental condition, the affected individuals experience symptoms of depression in winter while they get better as soon as spring and summer arrive.

To add to George, unfortunately mental conditions are VERY SUBJECTIVE and reliant on the patients description. Many mental conditions, including SAD, would otherwise be impossible to assess on a 15 min clinical consult. Similarly, that's why psychiatrists exist. Even GP / family doctors are (for the most part) clueless on mental affects. Psychiatrists are specialists trained for that. Psychologist would be good for therapy, and especially CBT. Psych+CBT actually has MUCH higher clinical support than anti-depressants for general depression Counciors would be good for ongoing therapy, and talking to someone. But only psychiatrists are experts at understanding and diagnosing. But to answer your question: if you live fairly north (north US or Canada), have depression symptoms (take a PQ-9 self-test), and ONLY happened as the fall approached: with NO outside influence that is causing a mental impact (such as stress, relationships, breakups), then it could be SAD or Vit D deficiency