It is also the date of the paycheck which is important.
When I was working, we had an odd thing happen back in 1986/87. We were paid on alternating Thursdays and 1/1/87 fell on a Thursday, a paycheck date. Of course, the office was closed that day and the next day (Friday), and payroll issued our paychecks (no direct deposit yet) on 12/31/86.
However, the paychecks were dated 1/1/87 because the big tax reform act passed into law in 1986 lowered income tax rates and the execs in my company would benefit by having their 1986 bonuses count as 1987 income and subject to the lower rates. So, we were issued paychecks on 12/31/86 but post-dated by one day, preventing us from cashing them that day. [Because we were all off from work on Friday, 1/2/87, we could go to the bank then.] We were given warning about this (without the underlying reason, of course) a few weeks in advance so we would not get blindsided at paycheck time.