An abbreviation for Tuesday; on schedules and calendars '火' stands for Tuesday.
会議は火に変更になりました（火＝火曜日）。
The meeting has been moved to Tue. (火 = Tuesday).
スケジュール表で『火』と書かれている日が休講日だ。
The day marked '火' on the schedule is a class holiday (Tuesday).
Fire as one of the five elements (Wu Xing) — the element 'fire' and the natural phenomenon of fire.
五行では火は夏や南、熱などを象徴する。
In the Five Elements, fire symbolizes summer, the south, heat, and so on.
キャンプでは火の扱いに十分注意する必要がある。
When camping, you need to handle fire with great care.