It was a tradition to open the White House on that day so the president could wish visitors a happy new year.

VOA: special.2009.10.08

These partnerships shouldn't be entered into lightly, warns Jamie Pedersen, a Washington state rep who pushed the registry and signed up for it with his partner, Eric Pedersen, the first day it was open in July.

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Unlike other European public transit systems, it stays open 24 hours a day, seven days a week.

BBC: Subway systems by the numbers

Some visitors use it as a base for hiking or heli-skiing (it's open only for day trips in winter), while many do nothing more strenuous than taking photos or reading on the deck, rousing themselves only for another estimable lunch.

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