The Christmas season can have a major impact on human health. Especially increased contact with in-laws during the holiday season is an important environmental factor known to affect both physical and mental health (Mirza et al., 2004). However, the mechanism through which in-laws influence host health is not yet understood. Emerging evidence has identified the intestinal microbiota as an important mediator for both physical and mental health. Here, we performed a prospective observational study to examine the impact of contact with in-laws on the gut microbiome during the Christmas season. We conducted 16S ribosomal DNA sequencing of fecal samples collected at two separate time points (December 23rd and December 27th 2016) from a group of 28 healthy volunteers celebrating Christmas. To discriminate between participants who visited their own family versus their in-laws, we built a multivariate statistical model that identified microbial biomarker species. We observed two distinct microbial-biomarker signatures discriminating the participants that visited their in-laws versus their own family over the Christmas season. We identified seven bacterial species whose relative-change profile differed significantly among these two groups. In participants visiting in-laws, there was a significant decrease in all Ruminococcus species, known to be associated with psychological stress and depression. A larger randomized controlled study is needed to reproduce these findings before we can recognize in-laws as a potential risk factor for the gut microbiota composition and subsequently host health.