A palindrome is a word, number, phrase, or other sequence of symbols that reads the same backwards as forwards, such as madam or racecar, the date "22/02/2022" and the sentence: "A man, a plan, a canal – Panama". The 19-letter Finnish word saippuakivikauppias, is the longest single-word palindrome in everyday use, while the 12-letter term tattarrattat is the longest in English.

Twosday is the name given to Tuesday, February 22, 2022, and an unofficial one-time secular observance held on that day, characterized as a fad. The name is a portmanteau of two and Tuesday, deriving from the fact that the digits of the date form a numeral palindrome marked by exclusivity or prevalence of the digit 2—when written in different numerical date formats, such as: 22/02/2022, 22/2/22 and 2/22/22. It is also an ambigram. According to University of Portland professor Aziz Inan, the palindrome is one of the "ubiquitous palindromes", as it retains its defining characteristics globally, despite the differences in national date formats. In countries that apply the ISO 8601 international standard for the calendar, there is an additional congruence inasmuch as Tuesday is the second day of the week under this scheme.