This is his answer why there is spoken about sabbath in plurial form.
tee mee'ah tohn sabbah'tohne -- the first of the shabbatot. Why "first"? Why plural "shabbatot"? Verse 6 provides the answer. Sha'ul apparently was in Philippi for Pesach. Just after Pesach, he traveled to Troas, a journey of about 400 km. It took him less than 5 days to complete the journey. There are 7 shabbatot between Pesach and Shavu'ot, that the Torah commands us to count off, during this period. That year, Pesach probably fell on a Shabbat or Sunday. Less than 5 days later, he arrived in Troas, and on the first of those 7 Shabbatot after Pesach, he met with brothers in Troas and shared a meal with them, and later taught them until after midnight.