前日 and 前夜: the preceding day and the night before
Compare preparing on the day before a presentation with checking a script the night before, preserving the narrower nighttime interval.
The day immediately before the reference date
前日, read ぜんじつ, means the day before an identified date. 発表の前日に資料を印刷しました。 says that the materials were printed the day before the presentation.
If departure is specified as Saturday, 出発の前日は金曜日です。 identifies Friday as the preceding day. Find the reference date before identifying the day before it.
The nighttime part of the preceding day
前夜, read ぜんや, means the night before the reference event. 発表の前夜に原稿を確認しました。 says that the script was checked the night before the presentation.
前日 alone does not specify night. 前夜 adds a nighttime interval to the relationship of being on the preceding day.
Different intervals within the same preceding day
前日の朝に準備し、前夜は早く寝ました。 says that preparations were made in the morning of the preceding day and that the person went to bed early that night. Keep morning and night distinct.
前夜に出発しました。 says that departure occurred the night before the reference event. It does not also supply an arrival date or journey duration.
Must the printing have occurred at night in 発表の前日に資料を印刷しました。?
Show the answer and why
No. It specifies the preceding day without specifying night. 前夜 would narrow the time to the night before.