Depends on whether that's one of your Evocation Elements. If you have Evocation (Fire, Air, Water) you nee to specialize in those, even if you've got Seelie Magic, since you're flavoring your existing elements with Summer, not using it as an element per se.
On the other hand if you're using an alternate element set and have Evocation (Summer, Spring, Autumn) you could grab Summer as a specialization whether you have Seelie Magic or not.