Birthdays of an Idol, a Group or their Fandom is considered an Anniversary and they are celebrated in the game through Anniversary Events.
An Idol Anniversary Event, better known as Idol Day (Such as Mingyu Day, Danielle Day etc.) starts one day ahead of their actual birthday. The event lasts for a week.
A 7-day login event is also released where you get a reward for logging in to corresponding amount of days
This is what a usual 7-day login rewards look like:
The SEVENTEEN Collection Tickets and Card Packs are for SEVENTEEN Members’ Days. Other Groups’ Member Days will reward you with their corresponding group’s Collection Tickets and Card Packs.
<aside> 💡 You can’t directly get the rewards of the 6th day without logging in for the past 5 days; You get rewards for logging in for set amount of days
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Special Idol Anniversary Concept Photo, Anniversary Profile Skin and Collection Photo becomes available for purchase in the shop. These can only be purchased with money. The Concept Photo and Profile Skin becomes available for purchase using Anniversary Collection Tickets later on (in an irregular basis).
Click this fandom webpage https://rhythmhive.fandom.com/wiki/Idol_day_event for complete info on Idol Days.