@trbilgin From what I understand, normally NVC schedules interviews once per month. This is a general practice across all consulates. See the "When Will I Get an Interview" paragraph in this document: https://travel.state.gov/content/dam/visas/PDF-other/NVC_role_in_IVs_for_applicants_November_2016.pdf
This was from 2016, but my understanding is that it generally remains true.
At the same time, we know from our friends in this group, that some months they schedule interviews more than once per month. For example, in December 2022, they sent interview letters (ILs) on Dec 5th, 7th, and 26th. But most were sent on December 5th. In January 2023, they sent ILs on January 2 and 27th, with the vast majority on the 27th.
My guess is that they schedule most interviews once per month, but add days if there are interview cancellations or unforeseen availability. In other words, the vast majority of ILs are sent once per month (almost always on a Monday or Friday), but they fill-in if there is extra availability.