Why are the four estimated tax due dates not evenly spaced across the year?

The IRS estimated-tax calendar splits the year into uneven periods rather than true quarters. Payments are generally due April 15 for income earned January through March, June 15 for April and May, September 15 for June through August, and January 15 of the next year for September through December. That means the second 'quarter' covers only two months and the fourth covers four, which trips up people who assume clean 90-day chunks. If a due date lands on a weekend or holiday, it shifts to the next business day. Mark all four on your calendar, because the compressed June deadline is the one people most often miss and get penalized for.