@Macelleriapescaiola - This one is an interesting challenge to setup, but I think I figured out a novel way to do this.  But a few assumptions to start:

• Orders Placed between Friday and Sunday get the Monday Shipping Email Notification the next day at 9AM.  Which means people who order on Sunday will get the notification about the delay on Monday.
• Orders Placed on Monday through Thursday always get marked as Fulfilled before Friday at 9AM (Fulfillment happens within 24 hours).

First, I would set a trigger on Placed Order as you have mentioned. Then I would put a Time Delay of 1 day but only set to be sent on Fridays, Saturdays, Sundays, and Mondays - then add your Message notification about the Monday Shipping.

Next, I would add a Flow Filter to Fulfilled zero times since starting flow.  

Here’s a quick screenshot of a Flow I whipped together quickly:

What’s happening here is all Placed Orders will technically start this Flow (including Monday-Thursday), but the Time Delay forces it to wait to be sent until the upcoming Friday.  However, once an order is marked as Fulfilled, they immediately exit from the Flow so they won’t receive the message as defined by the Flow Filter - Fulfilled zero times since starting this flow.

Since Placed Orders on Friday through Sunday aren’t marked as Fulfilled until Monday, they will get the messages within the next day (Note, in the screenshot I didn’t select Monday, but should if you want Sunday orders to get the notification too). 

I think this would work, but I’d leave it up to you to test it and tweak it. I would recommend you set this on Manual, look at the people who would have received the email and make sure all the logic matches your Fulfillment operations before you set this to Live.  And, you may want to consider some edge cases:

• If the Placed Order, was later Cancelled (or Refunded).  You might want to add that to the Flow Filter so they don’t get the Shipping Delay if they have cancelled their order before Fulfillment.
•  You may want to change the time of the Email Send to better match your operations or customer support availability.  I arbitrarily picked 9AM.
• You may have to tweak other Flow Filters for certain things that have longer processing time, or have no processing/shipping (e.g. Digital delivery of Gift Cards).  

Hope this helps get you started!