Fi can identify a full temporal interval while locating an act within some part of it. This must be distinguished from completing the whole interval.
Completion of the entire interval means performing what the wording requires across its whole duration. Occurrence within an interval may instead require only an instance at an appropriate time.
The contrast affects oaths, conditions and legal statements. A condition to fast the entire month is different from one to fast within that month: the latter may be met by a suitable instance of fasting during the period.