I'm somewhat inexperienced at investing, though I do have existing money in index funds. One piece I don't fully understand is the delay between order and execution and how I can estimate when exactly my order will be executed. For example, say I made a purchase at 9am this morning: I'm guessing this means I will get the price at noon the next day? Is this essentially locked in, and I don't need to worry about when it's executed? For example, if it didn't get executed until Friday (3 days later) for some bizarre reason, would I still get the price tomorrow at noon? Or is it a case of the execution is key, and it'll take whatever price it is at that time?
With traditional mutual funds, prices are calculated once per day when the markets close. When you make a buy or sell order, the order is executed at the end of the day when the next price is calculated. The price you get is the next available calculated Net Asset Value (NAV). If you place an order at noon on a trading day, you will get the new price at the end of the day.
Exchange-Traded Funds (ETF), however, are traded continuously throughout the day just like stocks. When you make a buy or sell order during trading hours, the trade is executed quickly. Like a stock, the price fluctuates during the day based on supply and demand, and can even be slightly different than the NAV.
For more information, see this article from Fidelity: Understanding how mutual funds, ETFs, and stocks trade