When using scheduled automation triggers, you must chose a date and time that is not in the past to be able to turn it on.
Given your examples, if you would like the first time the automation runs to be on 4/24/2023 at 9:00am, you could simply set the starting date to 4/24/2023 at 9:00am and turn it on. It would run on 4/24/2023, and the next time this automation would run would be 5/8/2023. If you would like 5/1/2023 to be the first time you would set it for 5/1/2023 and the next trigger would be on 5/15/2023.
If you set the trigger to say 4/20/2023 9am (today, and a Thursday), the automation will first trigger and execute on 5/1/2023 at 9am (ie two Monday's from the first set time).
I hope this helps you understand how this trigger behaves!