The resource for personal investors.
When you invest in ETFs, you'll notice that stock markets can move a lot in after hours trading. It's not uncommon for a market to open several percentage points up or down. You don't want to put a buy order in for an ETF, and it not get processed until the following day, as the price you pay can move massively versus what you expected.
The best time to make a purchase is in the morning. If your buy order gets processed by the early afternoon then hopefully the price won't have moved much, and you'll pay pretty much what you expect.