Friday:  2 pm to Midnight
Saturday:  9 am to 8 pm (just before the start of the Masquerade), and then after the Masq until Midnight
Sunday:  9 am to Midnight
Monday:  9 am to 2 pm 
Additional info: You are welcome to check any garments at the Coat Check. You may check boots, but only if they are in a plastic bag (to prevent leakage). All non-garments, including luggage, must be checked with the hotel.
If you are an Arisia member but do not have a reservation at the Hyatt, you may still check a bag at the hotel front desk. They will give you a ticket which you can use to reclaim your bag.