Shabbat, the Jewish Sabbath, starts at sundown on Friday evening end ends at nightfall on Saturday evening. Note that sundown occurs when the sun sets, while nightfall is defined by the appearance of 3 fixed stars in the sky (or about an hour after sundown if you can't see the sky). Jewish calendars are frequently printed with the time of day of each of these in larger Jewish communities, and nowdays, you can find apps that use GPS to give these times wherever you are.
The blessings of the Havdalah ceremony mark the end of Shabbat.
to honour the shabbat
The 3 stars in the sky say that Shabbat is over.
They celebrate Havdalah.
Every Friday at sundown
Religiously observant Jews would not board a plane if Shabbat were to start while in transit. The issue wouldn't be landing during Shabbat, the issue would be flying during Shabbat.
Every Saturday at sundown
Shabbat is the 7th day of the week, which is Saturday. All Jewish holidays start at sunset, so Shabbat starts on every Friday night and ends at Saturday night.
The spice box is not actually passed around until the very end of Shabbat. It is passed around toward the end of the concluding service of Shabbat, called Havdalah. One tradition explains that on Shabbat, each person is given an extra soul. The passing of the spice box at the end of Shabbat makes the loss of this extra soul a little less painful.
It is a Minhag (Tradition) that the woman of the house lights the Shabbat candles. (The reason for this is mentioned in the Talmud, that says that this is a Tikun (a correction, an amendment) to the "original sin" of Hava (Eve)) According to many Poskim (Rabbis) this act is a declaration of Kabbalat Shabbat (Welcoming the Shabbat) and thus the woman is usually the first to start the shabbat.
Friday at sundown
If a public fast falls on Shabbat, the fast is delayed until Shabbat is over. The one exception is Yom Kippur. If a public fast falls on Shabbat, you fast the entire day until the end of the fast, even though it means entering Shabbat while fasting.