If you do make it monthly, my recommendation is posting it on the 15th of every month, to stagger it with the other regular threads. This way you don't make the start of the month a scramble of postings.

Perhaps: Ask HN: What are you making?

How about at the end of every month?

So, people who start working on something new at the beginning of the month are ready to talk about their projects

It reminds me of Idea sunday / Project weekend or something like that