There were thirteen major strikes that took place at least in part in 2015 according to the Bureau of Labor Statistics monthly tables. This is just two more than in the year 2014, which had fewer major strikes which involved fewer workers than any year from 1947-2013 except 2009, a low point of the financial crisis. Most of the half century before WWII also had more major strikes (at least proportionate to the size of the population) than there are these days.
In all of the United States in 2014, there were just 11 strikes involve 1,000 or more workers, which involved a total of 34,000 workers and resulted in 200,000 work-days idle, which was less than 0.01% (i.e. less than 1 day in 10,000) of the total working time of the American labor force. The year 2015 was the next most peaceful year in post-WWII labor history.
The year 2010 also had just 11 major strikes, but those strikes involved more workers and produced more days idle. There were just 5 major strikes in 2009 in the United States which involved fewer workers than in 2014 and fewer days idle.
This has a lot to do with the decline of private sector unions in the United States. In 2014, just 6.6% of private sector workers (1 in 15) were union members and just 7.4% of private sector workers were represented by unions; in the public sector 35.7% were union members and 39.2% were represented by unions, with unionization rates in local government considerably higher than in the state or federal government. Just under half of union members are in the public sector and public sector unions, as a rule in the United States, have limited or non-existent rights to strike (although teachers unions which are a huge part of total number of public sector union members can frequently strike).
In 2014, just 0.001% of the working time of the American labor was idle due to strikes or lockouts. The last year than more than 0.01% of the working time of the American labor force was idle was 2000. The last year that 0.10% or more of the working tie of the American labor force was idle was 1978. Only one year from 1948 to 1959 , a time often nostalgically remembered as the "good old days" by conservatives was below the 0.10% threshold.
The labor department stopped keeping track of major layoffs of 1,000 or more people in May of 2013 due to budget cuts, but those are roughly 100 times more common than strikes and lockouts.