Online at SMO’s dictionary, (also my dear Green Dictionary) I thought I’d document the Gaelic and feel through what I should use for this.

For Midsummer, there is a choice of terms, naturally; there usually is in the Gaelic.

• Fèill Eòin
• Leth an t-samhraidh (Sàmhradh alone is ‘summer’, adding ‘an t-‘ is an article, ‘the’ and ‘leth’ means ‘half’, lit. ‘half the summer’)
• Là Leth an t-samhraidh (That’s Midsummer Day)
• Cuthach na gealaich-samhraidh (Midsummer madness, HA!)
• Maraiche (Midsummermere)

For Solstice, here are two:

• grian-stad (Literally, sun-stop)
• seas-ghrian (Literally, standing-sun)

My guess is, and someone correct me if I’m wrong, PLEASE, that ‘summer solstice’ would be ‘seas-ghrian sàmhradh’ and the winter solstice would be ‘seas-ghrian geamhradh’.