Regarding the 6/20 Unscheduled Maintenance

Attendance check for what?

From the Patch Notes:

 Spoiler

Attendance Check Event

June 19* - August 13

Requirements: Lv.13 or above

The Attendance Check Event is divided into 4 seasons, each covering a two week period.
Completing a full season of the event will earn special rewards.
Players participating in the event must accept the quest each day through the calendar notifier on the left, clicking on "Challenge" to start.
Defeat 300 monsters and turn in the quest through the star notifier.
Players will earn one Premium Dice for each Attendance Check completed.
Completing 7 days in a row will earn six Premium Dice and one Occult Cube, in total.
Players will also earn 5 Unleashed Coins for every day of the Attendance Check they complete.
[Update 6/20] Players will automatically receive the stamp for June 19, due to the patch delay.
Season 1: June 19* - July 2
Perfect Attendance Reward: Extra Character Slot Coupon
Season 2: July 3 - July 16
Perfect Attendance Reward(s): Weapon Enhancement Scroll, Armor Enhancement Scroll, Accessory Enhancement Scroll.
Season 3: July 17 - July 30
Perfect Attendance Reward: Incredible Chaos Scroll of Goodness (7 Day Duration)
Season 4: July 31 - August 13
Perfect Attendance Reward: Unique Potential Scroll (7 Day Duration)