The third day of the week, positioned between Tuesday and Thursday. The kanji '水' (sui) means water, associating this day with the planet Mercury. It's often considered the middle of the workweek.

週(しゅう)の真(ま)ん中(なか)の三(みっ)日(か)目(め)で、火(か)曜(よう)日(び)の次(つぎ)に来(く)ます。「水(すい)」の字(じ)は水(すい)星(せい)を表(あらわ)します。