Quoting from Wikipedia, the Mid-Autumn Festival is a harvest festival celebrated notably by the Chinese and Vietnamese people. The festival is held on the 15th day of the 8th month of the lunar calendar with a full moon at night, corresponding to mid September to early October of the Gregorian calendar.
People gift each other mooncakes leading up to the festival and have family dinners / celebrations on the day itself. And there are lost of decorations outside, mostly with lights and lanterns.
It is a really lovely festival and personally I compare it a bit with Thanksgiving as it is autumn related and family orientated.
This year I was in HK, so these pictures are from HK, not Singapore