1.
twenty days
A period of twenty days.
2.
twentieth day of the month
The twentieth day of any month.
二十日 has two uses: "twenty days" (a duration) and "the 20th" (a date). The reading はつか is irregular and must be memorized.
SPECIAL DAY READINGS (1st-10th):
Days 1-10 of the month all have unique native Japanese readings: 一日, 二日, 三日, 四日, 五日, 六日, 七日, 八日, 九日, 十日.
AFTER THE 10TH:
Most days use Sino-Japanese numbers + 日 (e.g., 十一日), but a few retain special readings:
- 十四日 (14th)
- 二十日 (20th)
- 二十四日 (24th)
COMMON COMPOUND:
- 二十日大根: small Japanese radish (literally "20-day radish" because it matures quickly)