The Vanguard Mid-Cap Index Fund ("VIMAX") is a portfolio that looks to capture the returns of the overall mid-cap stock universe. The fund's benchmark, the Center for Research in Security Prices (CRSP) U.S. Mid Cap Index, represents the universe of mid-capitalization companies in the U.S. equity market. The definition of what qualifies as a mid-cap stock can vary from person to person, but the generally accepted definition is a company that has a market capitalization of between $2 billion and $10 billion.

Mid-cap stocks are considered a bit riskier than their large-cap counterparts, but also carry the potential for greater long-term returns. Mid-cap stocks are designed to play a supporting role in an otherwise broadly diversified portfolio. The $24 billion fund has an expense ratio of 0.09%, making it one of the cheapest mutual funds available in the marketplace. The fund had delivered shareholders an average annual return of 11.5% for the five-year period ending Dec. 31, 2015. VIMAXX has managed to beat the average of Morningstar's Mid-Cap Blend category in four of the past five years, having narrowly missed beating the benchmark by 0.16% in 2012.

The fund has, historically, performed very well during its past five years, so positive gains throughout many of the individual months would be expected. An examination of the fund's monthly returns can give investors important information for how the fund has performed over different moments in time.

February Performs Best

The clear winner for the best performing month for the Vanguard Mid-Cap Index Fund has been February. Over the five-year period from 2011 to 2015, the fund has returned shareholders an average of 4.3% per month. That return is capped off with 6% returns in both 2014 and 2015.

Moreover, the month of February has been on a significant winning streak. The fund has delivered gains in seven consecutive Februaries from 2010 through 2016, posting an average gain of 4% in each month.

March Performs Well Too

While shareholders have enjoyed solid returns during February, they've also had the good fortune of the fund following it up with a strong performance in March. Over the past five years, the Mid-Cap Stock Index Fund has posted an average gain of 1.6%.

The month of March has seen a winning streak for mid-cap stocks as well. The fund has seen gains in March during seven of the past eight years. The average return during that eight-year period has been 3.4%.

Solid First Half Returns

The whole first half of the year has been pretty solid for the fund over the past five years. Outside of February and March, January has posted an average gain of 2.3%, while April has delivered an additional 0.5% average return. The month of May has been down 0.5% on average, thanks in large part to a 7.2% drop that the fund experienced in 2012. June has posted returns just a hair above breaking even.

Most Months Post Gains

The past five years have witnessed above average performance for the mid-cap universe, in general. In total, eight of the 12 months overall have averaged gains over the past five-year period. May, August, September and December are the only months to post losses.

August and September Struggle

For as strong as the fund has performed historically during the first half of the year, it has spent much of August and September giving back chunks of those gains. August, on average, has dropped approximately 1.4%, while September has lost an average of 2.2%. These two months also gave shareholders two of the three worst single-month performances over the 2011 to 2015 timeframe. In 2011, August posted a loss of 7% and was immediately followed by September losing another 9.9%.