The transaction date is technically the date on which your money was deposited into the company’s account. If you used a wire-transfer, the date of the transfer should be entered. If you wrote a check, the most accurate date will be the date of the deposit. However, if this is not available, use the date on which you wrote the check.
There are multiple reasons why this date can be important. First, if your money is within the first million dollars invested into the company, it might be considered 1244 stock (click here for more info). In addition, the date on which you invest can have a bearing on the preferences you get as an investor. For example, you may receive a better conversion rate on a Note if you invested before a certain date. Another reason why the date can be important pertains to Capital Gains. If you made an investment at the end of a calendar year, there could be tax ramifications if your check was deposited in the next calendar year.