src/Property/Event/RecurrenceRule.php
public function setByDay(string $day)
    {
        $this->byDay = $day;

        $this->canUseBySetPos = true;

        return $this;
    }
public function setByDay(string $day)
    {
        $this->byDay = $day;

        $this->canUseBySetPos = true;

        return $this;
    }
public function setByDay ( string $ day ) { $ this -> byDay = $ day ; $ this -> canUseBySetPos = true ; return $ this ; }
The BYDAY rule part specifies a COMMA-separated list of days of the week;.

SU indicates Sunday; MO indicates Monday; TU indicates Tuesday;
WE indicates Wednesday; TH indicates Thursday; FR indicates Friday; and SA indicates Saturday.

Each BYDAY value can also be preceded by a positive (+n) or negative (-n) integer.
If present, this indicates the nth occurrence of a specific day within the MONTHLY or YEARLY "RRULE".

@param string $day

@return $this
The BYDAY rule part specifies a COMMA - separated list of days of the week ; .