I don't think the term extraordinary is appropriate. It would be a meeting of the parish, not the parish council. Normally this is organised by the parish council and takes place on a specific date. NALC say The parish meeting of a parish must assemble annually on some day between 1st March and 1st June, both inclusive, in every year. In a parish which does not have a separate parish council, the parish meeting shall, subject to any provision made by a grouping order, assemble at least on one other occasion in the year. In other cases, subject to the aforementioned requirements, meetings of parish meetings shall be held on such days and at such times as may be fixed by the parish council or, if there is no parish council, by the chairman of the parish meeting.
If one has already been held in the Financial year, I don't think there is any legal reason why another can't be held, but the Parish Councillors might not be as willing to attend an additional one.