I am having problems with Firebird SQL statement in version Firebird 2.5.

Based on today's date, I have to select either this month's data or the previous month's data.

```
SELECT * FROM FA_DOBAVNICA
WHERE
1=1

AND CASE WHEN
    extract(day from cast('Now' as date)) < 9
THEN
   DATUM_NAROCILA BETWEEN 'start of previous month' AND 'end of previous month'
ELSE
   DATUM_NAROCILA BETWEEN 'start of this month' AND 'end of this month'
END 

```

I am getting a `104` error `Token unknown for BETWEEN`. I have no idea what am I doing wrong.