Guilherme, to be able to sort by date the field evento it is necessary to be with type datetime .

If the field is of type string, you will need to convert this date to a date instance and thus perform manipulation.

SELECT * FROM agenda ORDER BY STR_TO_DATE(evento, '%d/%m/%Y') ASC

Reference:

  

STR_TO_DATE (str, format)   STR_TO_DATE (str, format) This is the inverse of the DATE_FORMAT () function. You need a str string and a string format. STR_TO_DATE () returns a DATETIME value if the format string contains date and time portions, or a DATE or TIME value if the string contains only date or time portions. If the date, time, or datetime value extracted from str is illegal, STR_TO_DATE () returns NULL and produces a warning.
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Suggestion

It is recommended that if the field evento is type varchar() you switch to type datetime , as it is the right way and you will have many features available in MySQL to work with this field.

See more details on this other response: Varchar or Datetime?