Is a single expiation required, or an expiation for each day, from one who swore to pray Fajr in congregation upon hearing the adhan, but then failed to do so?

Expiation is required upon breaking an oath. A single expiation is obligatory for each day one intentionally failed to attend congregational prayer after hearing the dawn (Fajr) adhan (definition) or the alarm, because the object of the oath recurs daily. The breaking of the oath recurs if the oath, by its wording, the intention of its taker, or by custom, necessitates recurrence. An example of this is swearing not to abandon a continuous, recurring act of worship that has a specific time; in such a case, one breaks the oath every time they abandon it.