The last time the Supermoon was observed was in November 2016, when the moon was especially 'super', since it was closest to the earth it had been since 1948. Another Supermoon like that won't be witnessed until 2034.
Since the earth is closer to the Sun during winters in the Northern Hemisphere, the Sun's gravity pulls the moon closer to earth. As a result, Supermoons this time of the year appear closer than the ones during the rest of the year.
The Supermoon, however, is not an astronomical term and was coined by American astrologer Richard Nolle in 1979. According to timeanddate.com, a Supermoon occurs when the centre of the moon is closer than 360,000 km to the centre of the earth.
The best time to witness the Supermoon is right after moonrise. When the moon is just above the horizon, it appears brightest and largest. This is called the Moon illusion.