Definition of merger Monday
Generally, more mergers and acquisitions are announced on a Monday (typically before the stock market opens) than on any other day of the week, as the deal teams and merging companies have the entire weekend for last-minute preparations, public relations planning and other issues to finalise the deal.
This confluence of announcements is therefore referred to as 'Merger Monday' and can often dominate the business news during a particularly busy merger period. 
The fourth Monday of April 2012 set dealmakers’ hearts alight. Nestlé said it would buy Pfizer’s infant nutrition business for $11.85bn. AstraZeneca informed markets it would buy Ardea Biosciences for $1.3bn. And Thomson Reuters announced the sale of its healthcare unit for $1.25bn.
With more than $17bn in deals announced in a single day, observers heralded the return of “Merger Mondays”, which had been common around 2008. The flurry of activity sparked hopes that companies were finally shrugging off the eurozone crisis and getting back to doing deals.
But the enthusiasm was short-lived. The fourth Monday of April turned out to be an aberration and the months since have seen deal activity slow to a crawl.