tl;dr

• Patch 14.12 (June 12) to Patch 14.14 (July 15)
• Free Event Pass
• Special rewards including Treasure Tokens, Pengu
• Nostalgic game mode called Pengu’s Party
• New Tacticians including new Chibi Champion

Players can earn rewards just by logging in each day during the event and by earning XP on a new free 5-Year Bash Event Pass.

More details in this link