Research into the seasonality of mood has long been stymied by a lack of data, in part due to the prohibitive cost of traditional data collection and the tendency for data to be highly localized. Recent work using social media data has evinced the utility of psycholinguistic features in tracking mood and mental illness, but Twitter data, which are nonanonymous and short-form by design, have almost exclusively been the subject of analysis. In this article, we present a novel corpus within this field of study, comments from the social network Reddit, which does not suffer from these potential limitations. We find that although there are no notable changes in mood in the entire population over the course of a year, a small cohort is acutely sensitive to changes in the relative day length (i.e., the relative photoperiod). Our findings corroborate the phase shift hypothesis, which is the prevailing theory for the seasonality of mood. We also demonstrate the viability of the Reddit comments corpus for studies in mood and, more broadly, mental health.