廿 is a single-character ligature of
二十 ('two-ten') used in classical and lunar-calendar contexts, where it means '20' or 'twentieth.' Most famously in
廿四史 (Twenty-Four Histories) and in lunar dates like
三月廿六 (the 26th of the 3rd lunar month, as in Lin Juemin's 1911 farewell letter 《
与妻书》).
