Mid-Spring -- The second week of the second full month of spring.
Late Spring -- the last week of the last full month of spring.
Early Summer -- the first week within which summer begins.
Midsummer's Eve -- a magikal night indeed!
Late Summer -- the third week of August.
Early Autumn -- the first full week of autumn, to be exact.
The middle of Autumn -- sometime between the last week of October and the first week of November.
Late Autumn -- possibly the week before Thanksgiving.
Early Winter -- either during the week of, or just right after, Thanksgiving.
Right after the new year begins -- during the first week of January.