You can open your hosted feature layer in Pro, use Select by Attribute to select all records for each day, then use Field Calculator to replace them with their respective codes.
Alternatively, you can use an Arcade expression with some conditional statements to populate values in a field based on the existing values in another field. In order to use Arcade, make sure that sync is not enabled for your layer. You can use an Arcade expression like below.
If you need more details about this workflow, see this link How To: Populate a new field with values based on substring values from another field using Arcade e... .
if (Find("Monday", $feature.Day, 0)>0) {
return "1";
} else if (Find("Tuesday", $feature.Day, 0)>0) {
return "2";
} else if(Find("Wednesday", $feature.Day, 0)>0) {
return "3";
} else if(Find("Thursday", $feature.Day, 0)>0) {
return "4";
} else if(Find("Friday", $feature.Day, 0)>0) {
return "5";
} else if(Find("Saturday", $feature.Day, 0)>0) {
return "6";
} else if(Find("Sunday", $feature.Day, 0)>0) {
return "7"
}
Best,
MJ