If you're still wondering when will xiaomi release MIUI 12 update for Redmi Note 8 and Redmi Note 9 series, they haven't updated their own tracker page yet with any information regarding this.
https://c.mi.com/thread-3126620-1-0.html?t=1596389197887

So, asking about the official release date is useless because nobody knows :D

@MIUIUpdatesTracker | @XiaomiFirmwareUpdater