Why does 'Mon' not work when setting the date format to "ddd"?

Question

I'm trying to change the content depending on the day as you can see in the code below. When I set the format to "ddd" and write Mon, Tue, Wed etc. it doesn't work. However, when I change the format to "dddd" it works fine. I'm a bit concerned that it isn't working as described in the documentation. Am I missing something here?

%%[
SET @Day = FormatDate(NOW(), "ddd")


IF @Day == "Mon" THEN

    SET @SMSContent = "Hello!"
    SET @Link = "http://example.com"

ELSEIF @Day == "Tue" THEN

    SET @SMSContent = "Hello!"
    SET @Link = "http://example.com"

ELSEIF @Day == "Fri" THEN

    SET @SMSContent = "Hello!"
    SET @Link = "http://example.com" 

ENDIF

]%%


%%=v(@SMSContent)=%% %%=v(@Link)=%% 

Answer 1

I'd suggest writing it out the day to verify the documentation (it's not always right). Might be easier to test in the body of an email.

%%[

SET @Day = FormatDate(NOW(), "dddd")

output(concat("dddd: ", @Day))

SET @Day = FormatDate(NOW(), "ddd")

output(concat("ddd: ", @Day))

IF @Day == "Mon" THEN

    output(concat("(Mon) Hello! http://example.com"))

ELSEIF @Day == "Tue" THEN

    output(concat("(Tue) Hello! http://example.com"))


ELSEIF @Day == "Fri" THEN

    output(concat("(Fri) Hello! http://example.com"))

ENDIF

]%%

Answer 2

If your users are in time zones that cross the International Date Line (IDL)... Ie. One in US and one in Japan, you can find the first day of that days week and then add days to it and then test your expected date against the result.

To do this you would use:

Date myDate = Date.today();
Date weekStart = myDate.toStartofWeek();  //The result of this will return the week start
//day based on the Local, hence bad for users that may be across the IDL. 
Date d = weekStart.date().addDay(1);

If they are not going to be crossing the IDL, you can do it a bit easier.

// Cast the Date variable into a DateTime
DateTime myDateTime = (DateTime) myDate;
String dayOfWeek = myDateTime.format('E');
// dayOfWeek is Sun, Mon, Tue, etc.