Talk:Setsubun 2018/@comment-28137023-20180122102430/@comment-30480779-20180122132204

The Onsen is a one-time unlock after clearing a bonus quest (「発見 癒やしの秘湯」, Discovery: Soothing Secret Hot Spring) that unlocks during main quest progression. It also appears to be the second tab in the Event Shop, so don't worry abut using it up.