Waiting to invest on different days of the month?

I know traditional advice says "time in market beats timing the market". I get paid once a month on the first of the month and that's when my 403(b) contributions and employer match are taken out. I generally invest in my taxable brokerage on the 10th-15th of every month. My rationale is my 403(b) contributions will always capture the first of the month consistently and my post-tax contributions will capture a slightly delayed window consistently. I've heard there's typically a lull in the middl