April 13 (the Ides of April): The Roman shipping season officially reopens, particularly for transport of the annona (grain supply) from Alexandria in Egypt to the city of Rome. The evidence for this is an edict of Gratian from 380 CE directing African shipmasters (CTh.13.9.3.3).

The images above were taken by me at the Vatican Museums a few yrs ago. It is a 3rdC CE fresco from the Ostiense necropolis on the Via Laurentina, depicting the loading of food on a river boat from Rome's port at Ostia: http://www.museivaticani.va/content/museivaticani/en/collezioni/musei/sala-delle-nozze-aldobrandine/affresco-raffigurante-limbarcazione-isis-geminiana.html For the law: https://www.thelatinlibrary.com/theodosius/theod13.shtml

As many people point out to me every time I tweet shipping dates: these were rough temporal guidelines for sea transport & a terminus ante quem for grain shipping. See Lionel Casson's well-known article ( #OA now in @JSTOR): https://www.jstor.org/stable/4238693  & Sirks: https://www.worldcat.org/title/food-for-rome-the-legal-structure-of-the-transportation-and-processing-of-supplies-for-the-imperial-distributions-in-rome-and-constantinople/oclc/463833052