Time to have a look back
As subtitle suggests you have to look back at previous HACKvents and fill in matching parts of flags from given dates.
2014-12-01: HV14-BAAJ-6ZtK-IJHy-bABB-YoMw
2015-12-01: HV15-Tz9K-4JIJ-EowK-oXP1-NUYL
2016-12-01: HV16-t8Kd-38aY-QxL5-bn4K-c6Lw
Now we can assemble our flag.
HV17-5YRS-4evr-IJHy-oXP1-c6Lw