Use day name from DATE in another field as the key in a JSONB query in postgresql

I have a department table that has a jsonb field that give the hours of operation that looks like:

{
            "Friday": {
                "closeTime": "1970-01-01T17:00:00.000Z",
                "isOpen": true,
                "isOpen24Hours": false,
                "openTime": "1970-01-01T08:00:00.000Z"
            },
            "Monday": {
                "isOpen": false
            },
            "Saturday": {
                "isOpen": false
            },
            "Sunday": {
                "isOpen": false
            },
            "Thursday": {
                "closeTime": "1970-01-01T17:00:00.000Z",
                "isOpen": true,
                "isOpen24Hours": false,
                "openTime": "1970-01-01T08:00:00.000Z"
            },
            "Tuesday": {
                "closeTime": "1970-01-01T17:00:00.000Z",
                "isOpen": true,
                "isOpen24Hours": false,
                "openTime": "1970-01-01T08:00:00.000Z"
            },
            "Wednesday": {
                "closeTime": "1970-01-01T17:00:00.000Z",
                "isOpen": true,
                "isOpen24Hours": false,
                "openTime": "1970-01-01T08:00:00.000Z"
            }
        }

I have another table that associates some activity on a day with a department and I want to know if on the day that the activity happened is the department was open. So how do I query this jsonb field with the day of week of the date from the other table?

I have tried something like:

select a.activity_date, a.department_id, 
d.hours_of_operation->to_char(a.activity_date, 'Day')->>'isOpen' as isOpen
from activity a
join department d on a.department_id = d.id

but that only returns an isOpen value when the activity_date is a Wednesday (which is the last key in the hours_of_operation jsonb)

You probably overlooked blank-padding that comes with Day template:

DAY full upper case day name (blank-padded to 9 chars)
Day full capitalized day name (blank-padded to 9 chars)
day full lower case day name (blank-padded to 9 chars)

Which means only Wednesday doesn't get any spaces, because it's 9 chars long already. The rest end up as keys that don't match what's in your jsonb structure, because of the additional whitespace characters. The -> returns null in that case. _{Demo at db<>fiddle:}

select quote_literal(to_char(a.activity_date, 'Day'))
      ,a.activity_date
      ,a.department_id
      ,(d.hours_of_operation->to_char(a.activity_date, 'Day'))->>'isOpen' as isOpen
from activity a
join department d on a.department_id = d.id

Add a trim() and it'll be fine:

select quote_literal(trim(to_char(a.activity_date, 'Day')))
      ,a.activity_date
      ,a.department_id
      ,(d.hours_of_operation->trim(to_char(a.activity_date, 'Day')))->>'isOpen' as isOpen
from activity a
join department d on a.department_id = d.id

Or add a fill mode FM prefix in your to_char() template:

Modifier	Description	Example
FM prefix	fill mode (suppress trailing zeroes and padding blanks)	FM99.99

select quote_literal(to_char(a.activity_date, 'FMDay'))
      ,a.activity_date
      ,a.department_id
      ,(d.hours_of_operation->to_char(a.activity_date, 'FMDay'))->>'isOpen' as isOpen
from activity a
join department d on a.department_id = d.id