What it is
The Cron Expression Generator builds and reads cron schedules: pick the times you want and get the expression, or paste an expression and get a plain sentence describing when it runs.
Reading a cron expression
Five fields, always in this order:
minute hour day-of-month month day-of-week
0 3 * * * is "at 03:00 every day". */15 * * * * is "every fifteen minutes". 0 9 * * 1 is "at 09:00 every Monday".
The symbols are few: * means every value, , lists values, - gives a range, and / sets a step.
The mistakes that cost people a night
Day-of-month and day-of-week together are an OR, not an AND. 0 0 13 * 5 does not mean "Friday the 13th": it runs on the 13th of every month and on every Friday. To get one specific combination you need one of the two fields to be *.
*/7 does not mean weekly. Steps restart at the beginning of each field, so */7 on days fires on the 1st, 8th, 15th, 22nd and 29th, then jumps back to the 1st.
Timezone is the server's, not yours. A job set for 03:00 runs at 03:00 wherever the machine thinks it is, which is usually UTC.
Every minute is rarely what you want. * * * * * is easy to write by accident and will happily run your job 1,440 times a day.