These can be earned during an anniversary event Stage Quest in the Music Arcade. They are also purchasable through bundles in the shop during an anniversary of an Idol, a Group or a Fandom.
You can use these to purchase anniversary themed Concept Photos (profile picture) or Profile Skins. You can check out the catalog by going to Collection Book → Artist → Select an artist of your choice → Concept Photos/Profile Skins.
The first row is usually the Anniversary or “Birthday” theme as seen below:
An Anniversary Concept Photo requires 4 Anniversary Collection Tickets to be purchased
An Anniversary Profile Skin requires 10 Anniversary Collection Tickets to be Purchased
<aside> 💡 Not all Anniversary Concept Photos/Profile Skins will be available to purchase and will be later made available on an irregular basis.
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These Concept Photos give Card Stat Buffs and other Bonuses which you can read in detail here: