x1 is the warm period (season/interval) of year x2 at location x3.
This can be used generically in order to describe the warm half of the year on Earth (approximately the interval from the (local-vernal) equinox to the immediate next (local-autumnal) equinox such that the warmest solstice (local-Summer) is included); this meaning is what was meant by the English word "Summer" in the 1200's, for example.