Converting enum type to *enum type in go

go newbie here,
I want to convert an enum from Day type to *Mappings type, since they are strings, I am unable to assign pointer values to the enum directly.
I tried to assign the value to a temporary variable

var id = MON
*r = id

but that didn't work, I want it to be assigned to a pointer so that I can have nil values for the type. I can't change the Day struct or Mappings struct type.
How can I assign the value to the receiver *r without running into the pointer issue?
playground link: https://play.golang.org/p/5SNx0I-Prc2

package main

type Day string

const (
	SUNDAY  Day = ""
	MONDAY  Day = "MONDAY"
	TUESDAY Day = "TUESDAY"
)

type Mappings string

const (
	SUN Mappings = ""
	MON Mappings = "MON"
	TUE Mappings = "TUE"
)

func main() {

	type A struct {
		day Day
	}

	type B struct {
		day *Mappings
	}

	sourceObj := A{day: MONDAY}

	destObj := B{}

	destObj.day.To(sourceObj.day)

}

func (r *Mappings) To(m Day) {
	switch m {
	case MONDAY:
		*r = MON
	case TUESDAY:
		*r = TUE
	}
}

destObj.day will be nil. Hence, *r and *destObj.day will be a run time exception.
Allocate space for destObj.day by using the new keyword.
Example:

destObj := B{new(Mappings)}