Cocktail Summer is an event in which you collect Solid Ice from either being online or slaying boss monsters in instances. These ice cubes can then be either crashed with an Ice Hammer to receive an Ice Box or be exchanged for vouchers each week, which then can be redeemed at the last day of the event to receive the desired Ice box.
Event data in a nutshell:
Event start: 2nd July
Event end: 23rd July
Voucher NPCs: 23rd July – 30th July
Voucher NPCs include the NPC to redeem the voucher coupon for an Ice Box as well as the NPC who is also exchanging Solid Ice for a voucher.