data
is the name of your section, the field is call period
, so it should be
$field = $page->blueprint()->field('period');
$label = $field['label'];
As regards a dynamic label, you can query data from other locations
fields:
period:
label: "{{page.parent.someFieldThatContainsTheLabel}}"
type: text
This could also be done in a structure field where multiple labels could be defined