There are a few ways to go about this; one way is to return an array of values with a QUERY function (this assumes a header row in row 1, and you want the last 7 dates):
=QUERY(A2:B;"select B order by A desc limit 7";0)
and you can wrap this in whatever aggregation function you like:
=AVERAGE(QUERY(A2:B;"select B order by A desc limit 7";0))