Text Formula String Concatenation - Omit Trailing Comma

@AdrianLarson I used your suggestion for a similar solution and it worked for a year or so, until I was asked to include more Checkboxes (12 in all) in the formula, then I got:

Error: Compiled formula is too big to execute (5,713 characters). Maximum size is 5,000 characters.

I found a simpler solution. Include the ", " in each result, then add a period to the end of your IF statements. Wrap the code with SUBSTITUTE to replace ", ." with null in order to remove the trailing comma at the end of your text

SUBSTITUTE(
IF( Sunday__c, 'Sunday, ','') &
IF( Monday__c, 'Monday, ','') &
IF( Tuesday__c, 'Tuesday, ','') &
IF( Wednesday__c, 'Wednesday, ','') &
IF( Thursday__c, 'Thursday, ','') &
IF( Friday__c, 'Friday, ','') &
IF( Saturday__c, 'Saturday, ','') &
'.'),
', .', NULL)

I think this is how far you have to go to implement formula here. I'm doubtful its compile size will be under the limit, but I haven't checked.

IF(Sunday__c, 'Sunday' & IF(
    OR(Monday__c, Tuesday__c, Wednesday__c, Thursday__c, Friday__c, Saturday__c), ',', ''
), '') &
IF(Monday__c, 'Monday' & IF(
    OR(Tuesday__c, Wednesday__c, Thursday__c, Friday__c, Saturday__c), ',', ''
), '') &
IF(Tuesday__c, 'Tuesday' & IF(
    OR(Wednesday__c, Thursday__c, Friday__c, Saturday__c), ',', ''
), '') &
IF(Wednesday__c, 'Wednesday' & IF(
    OR(Thursday__c, Friday__c, Saturday__c), ',', ''
), '') &
IF(Thursday__c, 'Thursday' & IF(
    OR(Friday__c, Saturday__c), ',', ''
), '') &
IF(Friday__c, 'Friday' & IF(Saturday__c, ',', ''), '') &
IF(Saturday__c, 'Saturday', '')

---

A couple notes:

• You don't need to check Checkbox__c = true for a checkbox in a formula. Just use Checkbox__c.
• The proper string concatenation operator is a single ampersand (&).