When specifying a cron trigger, I used

0 0 20 * * MON-THU

It appeared to save ok, but when it didn't run the build I checked the bamboo.log file and found

2007-03-20 09:56:22,576 ERROR [SocketListener0-7] [CronTriggerBuildStrategy] java.lang.UnsupportedOperationException: Support for specifying both a day-of-week AND a day-of-month parameter is not implemented.
java.lang.UnsupportedOperationException: Support for specifying both a day-of-week AND a day-of-month parameter is not implemented.
at org.quartz.CronExpression.getTimeAfter(CronExpression.java:1234)
at org.quartz.CronTrigger.getTimeAfter(CronTrigger.java:857)
at org.quartz.CronTrigger.getFireTimeAfter(CronTrigger.java:587)
at org.quartz.CronTrigger.computeFirstFireTime(CronTrigger.java:836)
at org.quartz.core.QuartzScheduler.scheduleJob(QuartzScheduler.java:606)
at org.quartz.impl.StdScheduler.scheduleJob(StdScheduler.java:238)
at com.atlassian.bamboo.build.strategy.CronTriggerBuildStrategy.initialiseJob(CronTriggerBuildStrategy.java:100)
at com.atlassian.bamboo.build.HibernateBuildManager.createSchedulingJobs(HibernateBuildManager.java:384)
at com.atlassian.bamboo.build.HibernateBuildManager.saveBuildConfig(HibernateBuildManager.java:205)
<snip>

Changing the cron expression to

0 0 20 ? * MON-THU

worked ok. The first style is a valid cron expression (it came out of an existing crontab file). If it isn't valid for quartz, an error message should be shown.