Note that 先 can mean "last/in the near past" when it's not modified by any attributive phrase/clause.
e.g.
先の日曜日: the last Sunday
この先の日曜日: Sunday in the future
These two 先 have different pitch accents. The former 先 is type 2 /サキ]/, i.e., the accent is on the second mora, and the latter is type 0 /サキ=/, i.e., flat. The former one has the same function as 先月 or 先週, as Mike-san mentioned. Thus, この前の日曜日 / 先の日曜日and この先の日曜日 are opposite in meaning.