You need to collect 4 different weapon parts, they randomly drop. They kinda look like a fusion rifle, its called DVALIN and its a legendary. Take it to the gunsmith and when you have collected all 4, on the 7th of every month the mission will be there for you. The missions are pretty simple, you can look up the codes that you need and i also recommend you get a destiny public event tracker app to make the last part of the quest easier.