Time!

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Across
  1. 7. Happening every hour
  2. 8. Without exception; at all times
  3. 13. Occurring every two weeks; synonymous with biweekly
  4. 15. Occurring once a month
  5. 17. Occasionally, but not always
  6. 18. Occurring once a year
  7. 19. Not often; seldom
  8. 20. Now and then; from time to time
Down
  1. 1. Occurring every three months
  2. 2. Occurring every two months
  3. 3. Once every year
  4. 4. Occurring at a certain time of the year
  5. 5. At no time in the past or future
  6. 6. Occurring every day
  7. 9. Occurring once a week
  8. 10. Not often; rarely
  9. 11. Lasting forever; without end
  10. 12. For a very short time; briefly
  11. 14. Frequently; many times
  12. 16. Occurring every two weeks