Yosemite is all about standing beneath El Capitan, walking among giant sequoias, and catching the sunset turn Half Dome orange. Late spring brings roaring waterfalls while fall clears out the crowds for quiet hiking, but you have to grab your park reservations months in advance to make it happen.
A typical trip runs about $2500 for a week of lodging, transport, and food. Spreading that out over time helps you lock down campsite or hotel bookings early without wiping out your checking account all at once.
That 0% window is a hard deadline, not free money, so if any of the $2500 balance is still sitting there when the 18 months are up, you will get hit with interest on the whole amount.