The (recent) origin of the claim seems to have been this story in News Hound, entitled, "January 4, 2014 - Planetary Alignment Decreases Gravity - Float For Five Minutes!" Here's what they're claiming:
It has been revealed by the British astronomer Patrick Moore that, on the morning of January 4th 2014, an extraordinary astronomical event will occur. At exactly 9:47 am, the planet Pluto will pass directly behind Jupiter, in relation to the Earth. This rare alignment will mean that the combined gravitational force of the two planets would exert a stronger tidal pull, temporarily counteracting the Earth’s own gravity and making people weigh less. Moore calls this the Jovian-Plutonian Gravitational Effect.