Why Ramadan's date change every year?

Because the Hilāl, or crescent moon, typically occurs approximately one day after the new moon, Muslims can usually estimate the beginning of Ramadan;[53] however, many[who?] prefer to confirm the opening of Ramadan by direct visual observation of the crescent.[54]

Ramadan is a month in the Muslim hijri calendar. It's the month were Muslims fast from sunrise to sunset. The hijri calendar is a lunar calendar and therefore it shifts back 10-12 days every calendar year. Muslims confirm the first day of the month after they observe the new moon. And it ends as they observe the new moon for the month of Shawwal.