For Tantris' ending:
Year 2, Month 1 - Tantris is first encountered while adventuring. After a string of automatic events, he becomes hireable.
Year 2, Month 3 - Make sure to get 3 big stars on Assignment 5.
Year 2, Month 4 - Event between Tantris & Merideth while adventuring with Tantris in party. As long as you got 3 big stars on Assignment five, there will be another event when you return to town.
Year 3, Month 1 - Event occurs while leaving the workshop. Tantris will be unavailable for hire. After a few days, Tantris will appear at the workshop and be hireable again.
Year 3, Month 4 - Automatic event between Tantris and Merideth. Speak to Tantris to get a recipe. Synthesize this item for Tantris' final event. This will also unlock the Duet trophy.
The dates are fairly fluid, just around when they are first available.
At the end of Assignment 12, choose the Normal Ending option with Tantris. The Normal ending requires getting more than about 100 yellow stars on the assignments to be available. There is no reason to not max out the stars for each assignment in Rorona Plus.