on occasion—
1. sometimes; from time to time:
- Three, four, or more forms of the same gene have on occasion been encountered.
2. as need or opportunity arises:
- A member of the House of Lords … is prepared on occasion to assert his political rights.
on the occasion—
1. in this instance:
- The House proved willing to remove individual ministers and on the occasion the government itself.
2. in connection with some festive or solemn occurrence:
- Several promotions were made at St. Petersburg on the occasion of the Emperor’s name-day.