First, they do not change the value of a stock. They do change the price and the number of shares outstanding. At the most basic level, if a company has announced it will split its stock 2 for 1 and the stock is trading at $100 and you want to buy the stock, here's what happens and in this order:
Announcement Date: This is the day the company announces it will split its stock 2 for 1. Most splits occur for stocks when they reach a rather high price. Companies feel there will be more trading and better distribution of its shares if the stock has a lower price. The way to accomplish those two goals is to have more shares trade. So if they split the stock 2 for 1, there are twice as many shares outstanding, and the price of the stock will be cut in half, allowing more people to buy more of the stock. You do not need to buy the stock on the Announcement Date to receive the new split shares. You need to buy it before the Pay Date to get the split shares.
Record Date: This is the date companies use to determine who is entitled to the split shares. They look on their books to see who owns the shares as of this Record Date and then send the new shares on Pay Date to the owners of record on the Record Date. Any share owners who sell their shares after the Record Date but before the Pay Date will have their shares forwarded by their brokerage firm to the new owners of the stock. While the previous owners won't get the new split shares, they will get the higher price for their stock.
Pay Date: This is the most important date for receiving the split shares. If you don't own the stock by this date, you won't get the split shares. So you need to buy the stock before the Pay Date and then you'll get the shares. (Actually, you can buy the stock most of the time on the Pay Date and still get the shares, but don't wait until the last minute.) Let's say you want to buy a stock selling at $100 a share that has announced a 2 for 1 split. If you buy it before the Pay Date, you will pay $100 for 100 shares and have a total cost of $10,000 plus a brokerage commission. If you buy the stock after the Pay Date (and the stock stays at the price at which it splits - $50), here's what happens: you take the same $10,000 you were going to buy the 100 shares of stock with before it split and buy 200 shares of stock for $50 plus a brokerage commission.
Now let's compare your position before and after the split. If you bought the stock before the Pay Date, you paid $100 for 100 shares for a total of $10,000. You then received the split shares and when the split was paid to you, you would show in your account 200 shares (the company has sent you 100 shares of stock which gives you 2 times the number of shares you had previously) with a cost basis of $10,000 or $50 per share. If you bought the stock after the Pay Date, you paid $50 for 200 shares, and your account will show you have 200 shares with a cost basis of $50 per share for a total value of $10,000. Looks the same no matter when you bought the stock. And that's the point: it doesn't matter when you buy the stock in terms of getting the split or not. You still have the same value for the stock.
Ex-Dividend Date: This is the date which is the day after the Pay Date and means you no longer get the split shares. The stock is trading without (or ex-) the split shares available to the new buyer, at the new lower price, reflecting the stock split.
The basic message about stocks that split: Buy a stock because it has good investment value, not because it's going to split. The split won't make any difference to your stock unless the company it represents is a good one.