Guide from raultono10 at 15 Jan 2024. Link below.
https://www.trueachievements.com/a381460/welcome-back-achiev...First you need to be in the normal or true ending route, for this you have to choose some specific answers during December
“Wait a second here”
“We’re missing something”
“Namatame's true feelings”
“Something’s been bothering me”
“We’re missing something”
“Calm the hell down”
After that you'll have some time to
max out Marie's Social link which is required for the true ending, Marie will be more available during this month
eventually during February you'll have to clear another dungeon, achievement pops after beating the boss