Please consider enabling cron special time extensions by default. Having them enabled improves crontab UX. Grepping openwrt/openwrt suggest that enabling //BUSYBOX_DEFAULT_FEATURE_CROND_SPECIAL_TIMES// under openwrt/package/utils/busybox/Config-defaults.in should do the trick.

• Software versions of OpenWrt/LEDE release, packages, etc. Busybox 1.34.1-1, OpenWrt SNAPSHOT r18125-b764cb9e5b
• Steps to reproduce

1. Add a scheduled task via LuCI or crontab -e Example: @weekly foo.sh
2. Log: cron.err crond[25439]: user root: parse error at @weekly