Sunday is the only day ____ i can go shopping

问题描述:

Sunday is the only day ____ i can go shopping
这里为什么不能填THAT,不是说定语从句中先行词中有the only 就要用that么?

在这句话中,the only day 在从句中做时间状语,I can go shopping on the only day,并不能用that代替.