Hi RUDO,
As mentioned above, Timers are the mechanism in OutSystems to run automated logic adequate to our scenario.
However, you don't have "out-of-the-box" the possibility of scheduling it to the last day of the month.
A possible strategy is to make it run daily and check if it is the last day of the month. If it is, you run your logic, otherwise the timer finishes without doing anything.
Here's how it looks:

The last day of the month is calculated by getting the first day of the next month and subtracting one day like on the screenshot above.
The OML with the code follows in attachment.
Hope it helps.
Regards,
João