An individual who resides in a town for 30 days is “liable for contributing to the soup kitchen [התמחוי, tamhui], three months to the charity box [קופה, kuppah], six months for the clothing fund, nine months of the burial fund, and twelve months for contributing to the repair of town walls.”
— Babylonian Talmud, Baba Bathra 8a
Alternatively, it is 30 days to the charity box obligation and three months to the soup kitchen. This is the way Mishneh Torah [12th Century CE compilation by Moses Maimonides] records the obligation.
This set of questions from the American Jewish World Service probes the themes behind this ruling. Additional pages discuss “the universe of obligation.”
As part of NaBloPoMo (National Blog Posting Month), a cousin of NaNoWriMo (National Novel Writing Month), “A Song Every Day” plans thirty daily posts with some connection to the number 30.