User blog comment:Yankovic270/Earlier Opening of Japan/@comment-32656-20111017134848

Earliest possible time? Not much - picture their civil war from otl, going on for decades.

Past that, it varies by when. If in the 1700s, the civil war gets better. Any other time in the 1800s, you're just shoving events into play faster than otl.

Both Civil War cases - Christians v.s. the otl Factions, with as otl eventually happening slowly if the two win, and some sort of weakened Catholic Monarchy otherwise.