First, there arenโt any triggers to set the Halloween mode. The required expression is a restriction to the triggers. It basically tells the hub to only react to the triggers for setting this mode when Require Expressions are all true.
Secondly, you will need to look at the other modes (Evening?) to see what triggers them. You may have to set required Expressions on them so they donโt step on this one. If this in for Halloween lighting, you may only need to add a required expression to the Evening mode that is the counterpoint to the Halloween Required Expression (e.g., between Nov 1 and Oct 21).
Modes are really just virtual switches but only one can be 'on' at any one time.
Personally I use modes to change lighting levels throughout the day and use virtual switches to achieve subsets of the day.
For example I have Day, Evening, Night, Sleep and Away modes with light levels set for each mode. I then have 3 virtual switches, Workday, Worknight and Day Off which are set according to the state of the day.
I then use those switches as required expressions and conditions in other rules. Example: when I wake up I have the radio start, but only if Workday is turned on.