The RSPB say that the first Swallows should arrive in March (they don't say if that is early or late) as they fly in from their southerly winter feeding grounds.
We have our first Swallow. She (and I am being presumption here) arrived alone in advance of the rest. I expect, if the last four years are accurate as a gauge, for the rest to join her by the end of March.
It is rather sad in a way because we are now living in the buildings that the Swallows have had free run over for over forty years - although we still tolerate them nesting in the cavé.