This document announces a call for museums to participate in an international hackathon called Museomix, which brings together teams of developers, designers, and others to prototype new ideas for museums over a 3 day period in November. The hackathon will take place in 8 countries and 13 cities across 14 participating museums. Interested museums are encouraged to apply by April 13 with information on what roles they will play in organizing the local event. The document provides details on the application process and timeline for the Museomix hackathon.