Zmanim · Shabbat and the candles
Why do Shabbat times change every week?
Shabbat times change every week because they follow the setting of the sun, and sunset moves a little each day with the season and your latitude. In summer Shabbat enters late in the evening; in winter, early in the afternoon. It is never a fixed hour on the clock.
Shabbat keeps the sky's time, not the clock's
A fixed weekly hour would be simpler, but it would not be true to what Shabbat is. The day is tied to sunset — the candles before it, nightfall after it — and sunset is a moving thing. The earth's tilt lengthens and shortens the day across the year, and your position north or south stretches that swing further. So a Shabbat that begins late on a high-summer evening may begin in the early afternoon in deep winter. The hour changes; the anchor, sundown, never does.
Where this comes from
This is the plain astronomy of the Jewish calendar meeting the law of Orach Chaim 261, which fixes candle-lighting before shkiah (sunset). Because the mitzvah is bound to sunset rather than to a number on a clock, the time honestly has to move with the season. Far from being an inconvenience, it keeps the day rooted in the real sky over your own city.
What to do
Don't memorize one time and reuse it — check the candle-lighting and nightfall times fresh for the coming Shabbat and for where you actually are. A time that fit last month, or fits a friend in another city, will quietly be wrong for you.
Source: The astronomy of the Jewish calendar; Shulchan Aruch, Orach Chaim 261.
So each week's true times arrive without you having to track the sun yourself, let Kimá (the Jewish clock: your city's zmanim and candle times) update them for your place, Shabbat by Shabbat.
Open Kimá