Companies often hold negotiations during the weekend and prepare important business announcements for Monday. So many mergers and acquisitions were announced on Mondays in the 1990s that the term “Merger Monday” became popular by 1995.
 
Mergers became less frequent during the 2000s and 2010s, but the term “Merger Monday” is still part of the Wall Street lexicon.

 
The Free Dictionary
Merger Monday
A Monday on which several mergers and acquisitions are announced. Merger Mondays happen when the details of the mergers are finalized over the weekend and announced on Monday.
   
Google Books
The Economist
Volume 337
Issues 7939-7946
1995
Pg. 69:
IT MIGHT be called Merger Monday. On November 6th a total of $21.8 billion of deals was announced, including five mergers and acquisitions worth more than $1 billion each, in countries as far apart as Australia, Britain and America.
 
Google Books
The Economist
Volume 345
1997
Pg. 211:
Merger Monday
On a single day this week, Western Europe saw some $130 billion-worth of takeovers and mergers. Whatever the causes of Merger Monday, the region’s businesses will never feel quite the same again.
 
20 October 1998, Philadelphia (PA) Inquirer, “Three new deals point to smaller wave of mergers, pg. C1:
Although the volume of deals yesterday doesn’t rival “merger Mondays” at the height of deal-making earlier this year, the agreements represent a higher level of activity than took place over the past two months.
 
Google Books
Financial Markets for the Rest of Us:
An Easy Guide to Money, Bonds, Futures, Stocks, Option, and Mutual Funds
By Robert Hashemian
Lincoln, NE: iUniverse.com, Inc.
2001
Pg 203:
As an aside, you may have heard of the expression Merger Monday. This refers to the flurry of M&A activities that are usually announced prior to the opening of the stock market on Mondays, causing a higher than usual level of trading activity.  THe reason behin the Monday morning (or Sunday night) M&A announcement is that many times businesses take advantage of the weekend to hammer out the final details of the upcoiming M&A, and by Monday morning they are ready to make the announcement.
 
Zero Hedge
“Rule 48 Mondays” Are The New “Merger Mondays”
Submitted by Tyler Durden on 09/19/2011 09:20 -0400
Remember when yelling “Monday” is all it took for the market to surge on mutual fund pumping and/or cheap money M&A? Neither do we.
   
InvestorPlace
Merger Monday Is Making a Slow Comeback
A flurry of deals bodes well for M&A, which is at a recent low
By Dan Burrows, InvestorPlace Feature Writer |  Nov 12, 2012, 1:38 pm EDT
OK, so it’s way too soon to proclaim the return of Merger Monday, but a recent flurry of deal activity has to give the market hope — especially considering that the value of mergers and acquisitions has hit its lowest point since depths of the financial crisis three years ago.
 
Monday’s are traditionally the biggest day for companies to announce major deals, and for the first time in ages, this week didn’t disappoint.