Accepted answer

In the label -> formatter for xAxis, use dateFormat function to get the month and then use the substring function to get the first letter and return that letter as follows -

xAxis: {        
    type: 'datetime',
    labels: {
        formatter: function() {
            var monthStr = Highcharts.dateFormat('%b', this.value);
            var firstLetter = monthStr.substring(0, 1);
            return firstLetter;

See it on jsfiddle

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