Story related and unlocks naturally.
*** Spoiler - click to reveal *** After you make it back to the metro, you will go through some cutscenes and talk to the general. After that conversation is over, make your way upstairs and your son will playfully tackle you. A short conversation ensues, and after a lightbulb is placed, everyone in the room will bring in the New Year. The achievement unlocks after a couple more cutscenes.