The Attendance Rewards are based on a monthly calendar: there are 28 rewards in total and as long as you login 28 days out of 30 or 31 you can collect them all (it doesn’t need to be consecutive login days).
On the 1st day of each month, the Attendance Reward Calendar will reset with new rewards.
Some rewards can include Soulbound items like Spellstones or Mounts; reason is to avoid exploits where a player could create multiple characters to collect many Spellstones or Mounts and then trade them via the auction house.
Note that new players won’t be able to collect all the rewards during the first month, except if they joined at the very beginning of the month.