all days; every day in the period
Refers to all of the days within a specified event, schedule, or period. It is more limited and schedule-like than 毎日, and it does not mean 'all day long' in the sense of 終日.
Participation on all three days of the training is required.
This seminar will be held on all days.
このパスは会期中、全日使えます。
This pass can be used on every day of the event.
Smoking is prohibited on all days inside the venue.
終日 means 'all day long' (the whole of one day), whereas 全日 means 'every day' across the whole span of a multi-day period.
前日 (the previous day) is a homophone of 全日 (all days) written with different kanji, so learners must rely on context or kanji to tell them apart.
A Sino-Japanese compound of 全, 'all' or 'whole,' and 日, 'day'; the spelling transparently represents 'all days.'