It had previously been in discussion for an extension and the decision came today. Multiple drama insiders told Stars News on the 14th, "'You Who Came From the Stars' has been extended by one episode and will come to an end with its 21st episode on the 27th." Sources close to the cast also stated, "It has been decided today that ['You Who Came From the Stars'] will air its finale on the 27th and we were contacted to make room in our schedules."
The drama's production company HB Entertainment rep confirmed to Xports News on the 14th, "'You Who Came From the Stars' has been confirmed to receive a one-episode extension and will end with a total of 21 episodes."
The drama is currently set to air its 18th episode during its regular time slot on the Feburary 19 at 10 PM KST. However, the drama will possibly not air on the 20th depending on whether the Korean women's curling team makes it to the finals for the '2014 Winter Olympics'. So depending on the schedule of the Olympics, the finale air date for the drama is subject to change.