明けましておめでとうございます! (Akemashite omedetō gozaimasu, Happy New Year!) The 2010s are done and we're now in the 2020s. 心機一転頑張ろう！ (Shinki itten ganbarō, It's time to turn over a new leaf!)
Another thing that starts over in 2020 is the 干支 (eto, Chinese zodiac), and the cycle starts with 子年 (nezumi-doshi, the Year of the Rat). This kanji also introduces our first lesson of the 2020s, ネズミ (nezumi, rat) is often written in kana as its kanji, 鼠, is kind of complicated. You will note, however, that the first kanji used in 子年 is 子 (ko, child). The kanji came first, and there are several explanations as to how it came to be associated with the rat. One story says that the upper classes in the Shang Dynasty (starting in the 16th century BC) wanted a way for the common folk to understand the zodiac, so they associated the cycle with 12 animals. Rats were associated with fertility and therefore were assigned to 子. Another legend has it that 12 animals took part in a race in which the rat was victorious and therefore assigned the first cycle, 子年.
Either way, 子年 is associated with family and fertility, and if you were born in the Year of the Rat (1924, 1936, 1948, 1960, 1972, 1984, 1996 or 2008) then you are in for a lucky 12 months.