Esther 9:21
To stablish this among them, that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly,
Cross-reference
Esther 9:17
Historical context
Esther 9:17 recounts the Jews rested on Adar 14, establishing the first day of the feast.
Esther 3:7
Historical context
Esther 3:7 explains the origin of the name 'Purim' and why Adar was chosen — the lot cast by Haman.
2 Chronicles 30:5
Parallel
2 Chronicles 30:5 records a decree for annual Passover observance — a parallel to Mordecai's establishment of annual Purim days here.