If those new investors came in at, say, $15 per share, the stock will become a day bagger for them on the day it increases $15 in a single day, calculated using the closing price for that day.
Day bagger vs. spiffy pop: what's the difference?
If you've been around The Motley Fool for long, you probably have heard another term that sounds a lot like this: Spiffy popping. The difference between a spiffy pop and a day bagger comes down to usage. Spiffy pop is a verb, as in "XYZ Labs spiffy popped!" and day bagger is a noun, "XYZ Labs was a day bagger for me today."
In reality, you should use both terms together, although not interchangeably, since they are different parts of speech. Just like you'd say a spring sprung, you'd say a day bagger spiffy popped. (I hope that clears things up.)