After digging through cron documentation and testing various configurations, I've found that weekly cron jobs trigger based on system time, specifically when both of these conditions are met:

• The minute/hour specified in the cron expression matches current time
• The system's week counter has rolled over (typically Sunday midnight)

Most Unix-like systems consider the week to start on Sunday at 00:00 (midnight). This means a weekly cron job like:

0 0 * * 0 /path/to/script.sh

would execute at midnight between Saturday and Sunday. However, there's an important distinction:

@weekly /path/to/script.sh

The @weekly shortcut specifically means "run once a week at midnight on Sunday morning".

Different cron implementations may handle this slightly differently:

Here are some common weekly cron patterns with explanations:

# Run every Sunday at 3:15 AM
15 3 * * 0 /usr/bin/backup.sh

# Alternative using @weekly syntax (midnight Sunday)
@weekly /usr/local/bin/weekly_report.sh

# Wednesday at noon (not actually weekly, just showing the day field)
0 12 * * 3 /scripts/midweek_check.sh

Remember that the day-of-week field (0-6 or SUN-SAT) uses 0 or 7 for Sunday in most implementations.

To test when your weekly cron will actually run:

# For systems with systemd:
systemctl list-timers --all

# Alternative method using cron debug:
sudo tail -f /var/log/cron

The exact log location may vary between /var/log/syslog, /var/log/cron, or journalctl output depending on your system.

In Unix/Linux cron, weekly jobs (specified as 0 0 * * 0) start at midnight between Saturday and Sunday (Sunday 00:00). This follows the traditional Unix convention where Sunday is considered day 0 in the week.

The cron daemon calculates weekly jobs based on the system's locale settings. Here's how it works internally:

struct tm {
    int tm_sec;    /* seconds (0-60) */
    int tm_min;    /* minutes (0-59) */
    int tm_hour;   /* hours (0-23) */
    int tm_mday;   /* day of the month (1-31) */
    int tm_mon;    /* month (0-11) */
    int tm_year;   /* year - 1900 */
    int tm_wday;   /* day of the week (0-6, Sunday = 0) */
    int tm_yday;   /* day in the year (0-365, 1 Jan = 0) */
    int tm_isdst;  /* daylight saving time */
};

While Sunday is default in most systems, some locales consider Monday as the first day. You can check your system's week start:

# Check system locale settings
locale
# Output may include:
LC_TIME="en_US.UTF-8"  # Typically Sunday-first
LC_TIME="ru_RU.UTF-8"  # Typically Monday-first

Here are some common weekly cron patterns:

# Run every Sunday at midnight
0 0 * * 0 /path/to/script.sh

# Run every Monday at 3:15 AM (for Monday-first systems)
15 3 * * 1 /path/to/script.sh

# Alternative using @weekly (always Sunday)
@weekly /path/to/script.sh

For consistent behavior across systems:

# Explicitly set timezone in crontab
CRON_TZ=UTC
0 0 * * 0 /path/to/script.sh  # Will always run at Sunday 00:00 UTC