It's the oldest song in a European vernacular language to which both words and tune survive, dating from circa 1250.* You can hear it here.
The original Irish Samhradh, Samhradh (“Summer, Summer”) references Bealtaine—it refers to the traditional gathering and bearing-back of wild greens with which to deck the home—but around here we sing it at Midsummer's, the Bealtaine of the North.
I initially learned the song from my friend singer-songwriter (and Dianic priestess) Ruth Barrett; it was released, with original Midsummer verses, on her 1994 album, The Heart is the Only Nation.
I love Ruth's new verses, but thought I'd try my hand at rendering the original Irish words into singable English. Here they are, just as we'll be singing them on Midsummer's Eve on the highest hill in Paganistan, a-conjuring Summer in.
Summer, Summer
Summer, Summer, milk of the heifer
We have brought the Summer in
Yellow Summer, brilliant daisies
We have brought the Summer in
This is the Summer that shall come joyful
We have brought the Summer in
Yellow Summer from the Sun's bed
We have brought the Summer in
Summer, Summer, milk of the heifer
We have brought the Summer in
Yellow Summer, brilliant daisies
We have brought the Summer in
Holly, hazel, elder, rowan
We have brought the Summer in
Shining ash from Bhéal an-Átha
We have brought the Summer in
Summer, Summer, milk of the heifer
We have brought the Summer in
Yellow Summer, brilliant daisies
We have brought the Summer in
Bhéal an-Átha: vyawl an-AWE-ha
*Interestingly, the oldest song in English to which we have both words and tune—the raucous, rowdy Sumer is Icumen In—is also a song about summer. I suspect that there's a reason for this. Here in the First World, with our cars, supermarkets, and central heating, we've forgotten just how truly miserable Winter can really be.