Yes, 7 days a week. If there are more people there requesting walk-in permits than they have available, they have a drawing/mini-lottery.
It happens at 11 AM, and if you don't get a permit, you can hang around. As people with reservations come in to get their actual permit, if they have fewer hikers than on their reservation, those become available to you.
At 2 PM, people with reservations who have NOT come in or called ahead are declared no-shows, and there is again a drawing for those available slots.
On weekends, by the end of the day, the available permit slots are usually all taken. But if you get there at the 11 AM time and hang around, your chances are very good.