GDS's and mainframe airline reservation systems when created had limited disk space, so restrictions were placed on data bases. The data base of flights and PNRs had to allow for keeping at least 3 days of already flown trips (so today-3) plus future scheduled flights. Then, they needed a buffer to allow for possible problems occurring with software processes which cycle out old days and cycle in the new future day flights. So maybe 3 old days, plus a week buffer leaves 355 days remaining in the year. That's my guess.