Celestial solstice

The celestial solstice (often but not always capitalized: Celestial Solstice), a merging of ley energies, is a recurring event during which the barriers between the planes grow thinnest, leading to major historical consequences on multiple occasions.

Vecna harnesses the solstice
After Vecna achieved lichdom during the Age of Arcanum, he and his forces disappeared into the Shadowfell, conquered Thar Amphala, and built the dark tower Entropis. From this stronghold, he made use of the celestial solstice to expand his power, but his foes eventually figured out his secret and used it to interrupt his first attempt at attaining godhood.

Elemental sundering
During one celestial solstice about 900 or 1000 years before the Campaign of Vox Machina—possibly before or after the Calamity began—huge elemental rifts burst open across Exandria, causing terrible earthquakes, fires, and a flood of elemental monsters against which the peoples of the world struggled to defend. Though they eventually beat back the elementals, they were terrified of what would happen on the next solstice. The druidic Ashari, until then a single nomadic society on the continent that would later be called Tal'Dorei, made a covenant to split into four tribes to seal the rifts and watch over the major remaining portals, where they settled: the Fire Ashari keeping vigil at Pyrah, the Water Ashari at Vesrah, the Earth Ashari at Terrah, and the Air Ashari at Zephrah.

Elam Genar mentioned a similar-sounding event in which "the planes touched" and genasi began wandering across Exandria. According to her account, this happened "long before the Calamity".

The Icelost Years
Some time between 513 and about 528 PD, when the kingdom of Tal'Dorei was still new and recovering from a 32-year war, another celestial solstice occurred. Errevon the Rimelord, taking advantage of the solstice, opened a rift from the Frostfell, the elemental plane of ice, in the forest that would later be called the Frostweald in the middle of the continent. The surrounding lands were immediately plunged into an oppressive winter, and with an army of frost giants and sentient blizzards, Errevon conquered most of the continent in a few years, expanding the winter storm as he went. The recently founded Council of Tal'Dorei finally convinced Kraghammer and Syngorn to work together in an alliance to fight Errevon; with their combined might they battled the Rimelord's forces back through the portal to the Frostfell, and the Ashari druids helped seal the rift. The people of Tal'Dorei celebrated this victory for centuries to come with annual festivals on Winter's Crest; the festivities take place when the barriers between the planes grow thinner, but not as thin as during a celestial solstice.

Raishan plans Thordak's escape
As part of the deal Raishan struck with Thordak, she promised to help him escape from the Elemental Plane of Fire in exchange for knowledge that would save her. She initially planned to do so on a celestial solstice, but when she saw madness growing in him from his exposure to elemental forces, she accelerated her plans and broke him out on Winter's Crest instead.