Play most recent Mixmarathon mix from SLAM on media player

Hey there,

This is my first post on this community forum, I found the engagement of the whole community amazing. So I thought I would give it a try.

First some background information at the Dutch radio station SLAM they have got the MixMarathon once every week on Friday. People can listen them back at a specific url. (https://www.slam.nl/terugluisteren/)
Now there is this diskjockey which I really like to listen to but most of the times his mixes are broadcasted when I’m at school or I don’t have got the time to listen to it. So I often find myself listening to his mix on the Friday afternoons. His mixes are always broadcasted at 09:00 (9 am) on Fridays. Home Assistant is a great tool to play the mixes back on my Bose Soundtouch speaker.

Calling this service below in the developer service caller thingy works like a charm

    {
	"entity_id": "media_player.bose_keuken",
	"media_content_id": "http://audio.slam.nl/[SLAM%20LISTENBACK]%20Slam%20Listenback%2020181012%200900.MP3",
	"media_content_type": "MUSIC"
}

The radio station playback urls are formatted like this

http://audio.slam.nl/[SLAM%20LISTENBACK]%20Slam%20Listenback%2020181012%200900.MP3

where 20181012 is equivalent to YYYYMMDD (%Y%m%d)
and 0900 would represent 9 o’ clock in the morning.

My question now is could anyone help me out with an automation triggered by an input boolean turning on, which results in playing the most recent mix on my media player? So I guess there would be some kind of template which formats the last Friday (might be today) into the YYYYMMDD-format which would then be used in the url. Thanks in advance.