Tip 1: When the schedule specifies both date and weekday, they're combined with a logical OR, i.e. the job will run if current_minute == scheduled_minute && current_hour == scheduled_hour && current_month == scheduled_month && (current_day == scheduled_date OR current_weekday == scheduled_weekday).
Tip 2: Cron jobs can break. Don't rely just on email to find out because the root cause can be outside of cron. Monitor each cronjob with a watchdog timer.
Tip 3: Run your servers including the cron process in UTC timezone. Why?
Tip 4: Some cron implementations allow to specify years and seconds. However, cron is not the best tool if you need to operate at those levels, which is also why crontab.guru doesn't support them.
Tip 5: Don't use @reboot because it has too many issues. crontab.guru doesn't support it.
More tips: Best practices for cron
Man page: crontab.org