The periods are weekly in which at least 15120 blocks out of 20160 blocks have to be voted yes. The first days of this period started a little slow, that is why we are not on the 15120 yes votes. This was because people had to update their wallet to version 4.0.2.1 and this took about 1 - 1,5 day for most people to upgrade.

However, currently around 90% of the votes are yes votes. So the next period the 15120 yes votes should be reached easily. This could change if people start voting no of course.

The percentage you see on http://navcoin.org/segwit is the % of 15120 reached. So once that percentage hits 100%, 15120 yes votes are done.

NAV Coin has a 30 second block time, so a vote is cast every 30 seconds.
Total time of voting period(20160blocks) : (20160*30seconds)/60 /60 /24= 7 days

The current period is from 1008000 to 1028160. This means around 2000 blocks are left in this period. Even with a 100% yes vote that will not get us to 15120 yes votes.
Next period will be from block 1028160 to block 1048320.

Let us get the 15120 yes votes in the next period and lock in SegWit.

For more info and why you should vote yes (or no), visit https://navtechservers.com/guides/segwit/.