For stocks it seems to be easy to find dividend payment information. However, for mutual funds, like VDIGX, I am having an issue figuring out expected payout dates and frequency. Can someone point me in the correct direction?
Determine which fund company issues the fund. In this case, a search reveals the fund name to be Vanguard Dividend Growth Fund from Vanguard Funds.
Locate information for the fund on the fund company's web site. Here is the overview page for VDIGX.
In the fund information, look for information about distributions. In the case of VDIGX, the fourth tab to the right of "Overview" is "Distributions". See here. At the top:
Distributions for this fund are scheduled Semi-Annually
The actual distribution history should give you some clues as to when.
Failing that, ask your broker or the fund company directly.
On "distribution" vs. "dividend":
When a mutual fund spins off periodic cash, it is generally not called a "dividend", but rather a "distribution". The terminology is different because a distribution can be made up of more than one kind of payout. Dividends are just one kind. Capital gains, interest, and return of capital are other kinds of cash that can be distributed. While cash is cash, the nature of each varies for tax purposes and so they are classified differently.
Mutual funds generally make distributions once a year in December with the exact date (and the estimated amount) usually being made public in late October or November. Generally, the estimated amounts can get updated as time goes on, but the date does not change.
Some funds (money market, bond funds, GNMA funds etc) distribute dividends on the last business day of each month, and the amounts are rarely made available beforehand. Capital gains are usually distributed once a year as per the general statement above.
Some funds (e.g. S&P 500 index funds) distribute dividends towards the end of each quarter or on the last business day of the quarter, and capital gains once a year as per the general statement above.
Some funds make semi-annual distributions but not necessarily at six-month intervals. Vanguard's Health Care Fund has distributed dividends and capital gains in March and December for as long as I have held it. VDIGX claims to make semi-annual distributions but made distributions three times in 2014 (March, June, December) and has made/will make two distributions this year already (March is done, June is pending -- the fund has gone ex-dividend with re-investment today and payment on 22nd).
You can, as Chris Rea suggests, call the fund company directly, but in my experience, they are reluctant to divulge the date of the distribution ("The fund manager has not made the date public as yet") let alone an estimated amount. Even getting a "Yes, the fund intends to make a distribution later this month" was difficult to get from my "Personal Representative" in early March, and he had to put me on hold to talk to someone at the fund before he was willing to say so.