The Celtic Summer Solstice
Last year, Celtic Source did a 5 part video series about the Summer Solstice, and if you missed it, I highly recommend you check it out.
Per the 1st video description: "Why is there no known Celtic myth for the Summer Solstice, the longest day of the year? Unlike Samhain, Imbolc, and Calan Mai, midsummer seems absent from ancient tales. But perhaps its myth was never lost, just hidden in the gestures and customs passed down through generations."
He lays out what we do know, what we don't, and how he's done some comparative mythology to sketch out a plausible myth. Even if you prefer to stick to the more definitively preserved celebrations, I think it's an excellent example of the process and level of work involved in filling out missing pieces of a practice.