What is the Sothic year?
The Sothic year originates with ancient Egyptians. Early Egyptian astronomers discovered that the start of the annual Nile floods coincided closely with the first visibility of the star Sothis, now known as Sirius, in the morning sky (known as the heliacal rising). It is likely that the Egyptian calendar year started on the date of this rising. The Egyptian year was divided into three seasons of four months each. Every month consisted of 30 days. At the end of the year, five additional days were added. Thus a year had 365 days.
However, with the calendar year having invariably 365 days, the calendar shifted one day every four years with respect to the heliacal rising of Sirus. It seems very unlikely that the shift of the calendar was not recognized by ancient Egyptian astronomers. Nevertheless, no documents of any serious attempt to correct the calendar have been found yet that are older than the Canopus decree of king Ptolemy III. Euergetes (r. 246 BC-222 BC). By this decree, in 238 BC, Ptolemy ordered an additional day to be added to every fourth year. But in practice the calendar remained largely unchanged.