I would have thought Friday or Saturday nights, but I guess coming home from work after the dreaded Monday is when you're the loneliest? Okay, that's me not even paraphrasing but just making up my own information. According to the actual research by Tinder, they looked at user activity over the past year and found that people were the most active on the app on Mondays around 9 p.m. The research also showed that the months in which Tinder was the most popular were January and August, and there were huge spikes in usage around music festivals like Bonnaroo and Coachella.
Not sure if this information will change your dating life in anyway, but hey, maybe it will give you something to look forward to on Mondays!