Meaning: Festival of Summer
When: The full moon closest to the Summer Solstice. This will be the full moon during either Samonios or Dumanios, whichever is closest to the solstice that year. Normally the latter. For reference: https://www.coligny-app.com/
Deity in focus: Taranis. This is the higest point of Samos and light in the year, representative of the victory of the light over darkness, summer over winter, Taranis over Andenamatos (or whatever one might call His enemy). So, as the deity perhaps most associated with Samos and Albios, this holiday is most appropriately fitting for Him.
This is also a time of storms, with Taranis flexing His might over His foes. An offering to Him is appropriate and this is His time of the year. Some cultic influences on Him in my Tegoslougos would be Thunaraz, Jupiter, and Mars, more so the former. To a lesser extent, possibly An Dagda of Irish lore, who though not specifically a thunder god, has a lot of traits that intuitively seem to apply to Taranis. At least to me.
Why it matters: This being the time of the triumph of Samos and its importance is likely rather self evident. Since we cannot live without a growing season. As we cannot live without food. So sun and rain, both of which are relevant to the sky are doubtlessly important.
Ways to observe: Other than the offering to Taranis, there are many ways to observe this holiday. Such as a feast, perhaps outdoors, maybe even a cookout. This is certainly a time of togetherness and enjoyment. Many places have various festivals this time of year, and there is much to go out and do. A good fire is yet another way to enjoy this important holiday.