I looked at the link, it looks really cool!!

But what do they actually do on groundhog day?

We don't have anything quite like that in the UK.

Ya Phil is a groundhog (very large but cute rodent). But no, they don't live that long.
The tradition is that old tho, and every few years they get a new groundhog and name it Phil.

It's based on an old superstition. People wanting an end to winter would lookd for signs of the coming spring. Someone probably saw a groundhog(or woodchuck, as wee call them in some areas) awakened early from it's hibernation on a cloudy day, and spring just happened to arrive 'early' ,,,they put 2 & 2 together and came up with 5. LOL
So now it's a tradition...(Sometimes right & sometimes wrong) If Phil doesn't see his shadow, spring will arrive early, about 6 weeks.
If he DOES see his shadow, he goes back to his burrow because there will be 6 more weeks of winter.

There's no set celebration for groundhogs day,.... not like Christmas or Easter. But people look at it as 'Closer to Spring'...and it actually cheers them up knowing that winter will soon lose it's grip.
A lot of business use it for a day of special sales n such.("Don't miss out on the Groundhog Day Special...20%off Sale!) LOL

We're a little closer to Thanksgivong Day - Nov 27th this year. I recently found out there use to be a holiday in the UK celebrating harvest, so it isn't completly American. There is an old Pegan celebration of harvest.