Here's a calendar of events I maintain for my own reference: conferences, workshops, hackathons and gatherings related to Open Development.
You can add it to your own calendar application, and if you'd like to collaborate: I'd be happy to share more relevant events! A short link: http://bit.ly/opendev-events
Disclaimer: I cannot guarantee all these events are correct and legitimate. I do some basic checks and provide a link to information, and would be grateful to receive corrections.
I've used different platforms to maintain lists of events in the past, most notably upcoming.org and Lanyrd. Our Open Development guide on Lanyrd worked for some time, but by mid-2016, a few years after the site was bought by Eventbrite, it became unreliable, and I've had occasions where I couldn't get the information out of it when I needed. Also, it seems that the collaborative nature of Lanyrd is declining, as it was with Upcoming before.
So I've moved everything into a Google Calendar, to serve my personal needs. To paraphrase Maciej Cegłowski's words from pinboard.in: "Open Dev events is a calendar for introverted people in a hurry."