Create subitem based on Calendar date

[wscharenbroch (Wendy Scharenbroch)]

I need to be able to create a subitem on the last day of each month. (Or first day of month if I cannot build an automation based date.) I have about 150+ items, on the last day of each month I need to add a subitem. I am relatively new to Monday, so any help would be greatly appreciated. Thank you!