This means "We will do as many things as we can on this day to make it happy and memorable." You could say "We'll make an occasion of it" but "We'll make a day of it" is the best way to say this.

Neutral/Standard We’ll spend the whole day there. We’ll turn it into a full-day plan. We’ll spend the day doing that. Informal options We’ll make a whole day out of it. We’ll make it a full-day thing. We’ll make a day trip out of it. We’ll spend the day together doing that. I hope you find this helpful, Anna