Putting it very simple, the price for a stock is mostly affected by the demand. There are other factors in play as well, but let's assume for this question it was actually that simple.
For ETFs, there is a mechanism called savings plan. At the start of every month, thousands of people automatically make a small investment into an ETF, thus increasing the demand (significantly?). Does this also affect the value of said ETF? I could think of one reason why this may not be the case, but I really don't know: It does not affect the price because the ETF resembles something else, e.g. through replication, and these values are not affected by the demand for the ETF. This also implies that an ETF is not affected by demand for the ETF itself, but only for the underlying values it tries to represent.
Is this intuition right? Or is it actually smart not to buy at the beginning of every month, because demand does indeed change the value of an ETF?