Twilight paused, and said in a subdued tone, “Estrus based on the day length, you know?”

“I... sorta,” Sweetie said. “As summer days grow very long, mares will hear the siren’s song.”

“Well, my friends and I,” Twilight explained, “This winter solstice, the sun didn’t rise. It was supposed to be the longest night of the year, but instead it lasted... forever. One week, three days and 4 hours, roughly. We saved Princess Luna and brought back the sun, but... it threw the whole season cycle off, and to keep us from having horrible blizzards, Princess Celestia was forced to keep the sun up longer to compensate. So, it was either a stronger estrus, a <i>longer</i> estrus, or blizzards.”

“Wow,” Sweetie said in honest awe, “I didn’t even think the long night would have meant that. Did it really last that long?”

“Time wasn’t passing normally in any case, but that’s my best estimate,” Twilight said with a strained smile. “If I could have figured out the location of the Elements earlier, or made friends quicker, it might have been different. I was just... really stupid about it.”

“Don’t say that,” Sweetie said in consternation. “You saved the <i>world!</i> Making mistakes is okay, if you do something like that! It wasn’t <i>you</i> who made the night last so long!”