Currently there is a event that allows us Halo fans to acquire some Mjolnir Driver Gear, yet I have not received my code for set suit. I have definitely played Halo 5, Halo MCC and Halo Wars 2 before Nov 30th and have booted up those games before Dec 31st yet I have not received any code for doing so…
It be a shame to miss out 
Probably want to get in contact with Xbox Live about this, they generally handle these types of promotions. Check the XBL Support forums as there may be a topic or 2 about it over there