It is possible to only participate in one day of combat, that simply means the player would negate one of the two days of battle damage.
Example:
(Day 1 is not participated in = Day 1 damage = 0) + (Day 2 = (day 1 damage (0)*.25 = 0)+ 10)
This would then mean that: Day 2 = 10.
However, we will disclose at this time that (spoiler) it may end up being difficult to avoid battle considering the scope and severity of combat that will occur at this time. Now, we're not saying it's impossible, as nothing is ever impossible! And we encourage players to do whatever they wish with their characters.