According to an announcement today, Caves & Cliffs, one of the most requested updates in Minecraft history (announced in October), it will come out in two parts.
The first part, which the accompanying FAQ states will include "cute and fun mobs, cool items and new blocks", will arrive this summer. The second part will come out instead during the Christmas holidays 2021 and will include radical changes to the generation of the world, a feature that gave the update its name, Caves & Cliffs.
Specifically the update will add new cave biomes, real mountains and various new building blocks.
The update was originally announced during Minecraft Live 2020, with a release date scheduled for summer 2021.
From what we read it is likely that the "meat" of the update will be in its second half, but apparently the programmers have opted to release some features first and make players happy without making them wait too long.
