You can use the field event triggers property to accomplish your goal.
1. Open Document Properties(Ctrl+Alt+D)
2. Click on Triggers tab
3. Go to the field event trigger, click on the 'Month' field and add trigger to it.
4. The required trigger in your case would be 'Select Possible'
Hope I am clear with my answer.
Thanks and regards
Our requirement is when month gets selected, its associated Date should get selected automatically(i.e without having to select date manually). Because we have one date in every month, it should be selected when month selected.
If there is only one possible date per month, there is no need to explicitely select this value.
Why not just rely on the associative data model? Selections should reflect user actions.
Or maybe I am missing something else?
If you absolutely need to select the dates, you can use a field trigger on Month field and a Selection - Select in field action to select the single possible value (or maybe a Select possible action or someother action).