Here "first" is the ordinal number one and "fifteenth" is the ordinal number fifteen. This is the first month of the Hebrew calendar. The fifteenth day is near the beginning of April on Western calendars. (See: [[rc://en/ta/man/translate/translate-hebrewmonths]] and [[rc://en/ta/man/translate/translate-ordinal]])
Yahweh proving that he is more powerful than all of the false gods that the Egyptians worshiped is spoken of as if Yahweh punished those false gods. AT: "he also proved that he is greater than their gods" (See: [[rc://en/ta/man/translate/figs-metaphor]])