BUG #14605: round trip of day of week doesn't work

The following bug has been logged on the website:

Bug reference: 14605
Logged by: Shaun Cutts
Email address: shaunc(at)factfiber(dot)com
PostgreSQL version: 9.6.0
Operating system: MAC OS
Description:

select extract(dow from to_date('Monday', 'Day'));
date_part
-----------
6
(1 row)

# select extract(dow from to_date('Sunday', 'Day'));
date_part
-----------
6
(1 row)