“Amateurs open the market, but professionals close it” is a stock market adage that’s been cited in print since at least 1993. Amateurs submit their trading orders in the morning (after spending the afternoon and evening analyzing events with stockbrokers and family members), but professionals follow market activities throughout the trading day and lock in their positions at the close of trading.
 
Electronic information means that the same information is instantly available to both professionals and amateurs at any time of the day, so the saying is somewhat anachronistic in the Internet age,
 
 
Trading Resource
Trading Wisdom
(...)
The open belongs to the amateurs and the close belongs to the professionals.
     
Google Books
Trading for a Living:
Psychology, trading tactics, money management
By Alexander Elder
New York, NY: J. Wiley
1993
Pg. 71: 
Amateurs (open)
Professionals (close)
(...)
Figure 18-1. The Meaning of a Bar Chart.
Opening prices are set by amateurs, whose orders accumulate overnight and hit the market in the morning. Closing prices are largely set by market professionals, who trade throughout the day. Note how often opening and closing prices are at opposite ends of a bar.
   
Google Groups: misc.invest.canada
Newsgroups: misc.invest.canada
From: Ted Carter

Date: 1995/09/22
Subject: Re: Successful Stock Market Speculation
 
I am now reading a book called Trading for a Living by Dr. Alexander Elder. As a result of observation and experience, I always knew that the first hour and last hour of the day were the two most active trading periods. I never really knew why but Elder points out that amateurs open the market and professionals close it. This makes a lot of sense to me. So don’t get caught up with the holdover momentum from the previous day, hold off and go with the pros and watch what they are doing throughout the day.
   
Google Books
Electronic Trading Masters:
Secrets from the pros
By Allen Jan Baird
New York, NY: Wiley
2001
Pg. 68:
I think this is a reflection of the old cliché that amateurs open the market and professionals close it, and at that time I was trying to trade the open like a professional.
   
Google Books
The Candlestick Course
By Steve Nison
New York, NY: John Wiley
2003
Pg. 12:
An American market adage contends, “The amateurs open the market, and the professionals close it.”
   
Google Books
Short Term Trading in the New Stock Market
By Toni Turner
New York, NY: St. Martin’s Press
2005
Pg. 141:
Hot Tip: You’ve surely heard the market axiom “Amateurs open the market, professionals close it.” Japanese and Western technicians agree: the closing price represents the most important price of the day. Why? Because the closing price broadcasts the commitment level of those who are willing to hold positions overnight.
 
Google Books
The Market Guys’ Five Points for Trading Success:
Identify, pinpoint, strike, protect, and act!
By A. J. Monte and Rick Swope
Hoboken, NJ: John Wiley & Sons
2008
Pg. ?:
There is an old saying that the amateurs open the market each day, while the professionals close the market.
 
ASX Market Watch
Aug 17, 2009
Amateurs Open, Professionals Close The Market
By Dave McLachlan
(...)
There is an old saying that the “amateurs open the market, and the professionals close it”.  This means that most amateurs are rushing in their orders at the opening bell, and most professionals are placing their orders in the last hour of trade.  And that makes the closing price extremely important.  If a stock travels down during the day, but in the last hour of trade comes back up to its highs, what could we assume?  That professional players (or players with large amounts of money) are buying the stock and raising the price.  Likewise, if a stock travels down but stays down until the close, we could assume that there are either no professional players buying, or there are professional players selling.
 
Leigh Drogan
The Open is for Amateurs, the Close is for Pros
Posted by Leigh Drogenon December 4th, 2009
Three straight days now the amateurs have painted the tape in the first hour, only to see the market lose VWAP and crater.  This isn’t healthy action people.  Institutions don’t do the bulk of their trading in the morning, it takes place in the afternoon after the dust has settled and their traders have a better chance at a positive P&L day.  Amateurs and retail investors do their trading in the morning.  There is an old say, the amateurs decide where the market opens, but the pros decide where it closes.  This is why technical analysts often point to a closing level and not intra day levels being more important.  We are seeing a very visual representation of big money handing stock over to small money, retail investors.