。
I'm spending Mother's Day on my mom.
Literal
Mother's-day [obj-を] mother [for-のために] use.
使う here stretches beyond 'use' into 'spend/devote' — you don't literally 'use' a day, you allocate it to something. The pairing 日を使う is a quiet reminder that 使う covers spending money, time, energy, and languages, not just physical objects. ~のために marks the beneficiary: 'for mother's sake.'