It is not established when exactly Katherine became John of Gaunt's mistress. Jean Froissart wrote that the love affair began during the lifetime of Blanche of Lancaster and Hugh Swynford, but he sometimes made mistakes in his chronicle (in particular, he pointed out that Katherine gave birth to three children from John of Gaunt, although there were four of them). Therefore, according to Alison Weir, his testimony cannot be considered reliable, because the people who provided him with information could be wrong. Also, some later studies indicate that the love affair between Katherine and John of Gaunt began during the life of her first husband, and her eldest son Thomas Swynford was "a child from two fathers", but this seems unlikely to modern researchers. In the document of the grant of an annuity to Katherine Swynford by King Richard II dated 7 June 1392, it is indicated that her first-born son with the Duke of Lancaster, John Beaufort, was 21 years old. Based on this, he must have been born between June 1371 and June 1372. However, Alison Weir doubts that he could have been born at this time, as from June 1370 to November 1371 the Duke was in Aquitaine, where in September 1371 he married Constance of Castile. Although it is possible that Katherine could have joined her husband in Aquitaine, Weir considers this unlikely as wives rarely accompanied their husbands to war. In addition, someone had to manage the estates in England and take care of small children during Gaunt's absence. In a petition to the Pope dated 1 September 1396, John of Gaunt indicates that the affair began when he was married to Constance of Castile, and Katherine was free from marriage. Therefore the affair, according to him, began after the death of Katherine's husband. Based on this, Weir concludes that most likely the love affair began in the late autumn of 1372, when there was a significant increase in Katherine's social status in the Lancastrian household.
It is not established when exactly Katherine became John of Gaunt's mistress. Jean Froissart wrote that the love affair began during the lifetime of Blanche of Lancaster and Hugh Swynford, but he sometimes made mistakes in his chronicle (in particular, he pointed out that Katherine gave birth to three children from John of Gaunt, although there were four of them). Therefore, according to Alison Weir, his testimony cannot be considered reliable, because the people who provided him with information could be wrong. Also, some later studies indicate that the love affair between Katherine and John of Gaunt began during the life of her first husband, and her eldest son Thomas Swynford was "a child from two fathers", but this seems unlikely to modern researchers. In the document of the grant of an annuity to Katherine Swynford by King Richard II dated 7 June 1392, it is indicated that her first-born son with the Duke of Lancaster, John Beaufort, was 21 years old. Based on this, he must have been born between June 1371 and June 1372. However, Alison Weir doubts that he could have been born at this time, as from June 1370 to November 1371 the Duke was in Aquitaine, where in September 1371 he married Constance of Castile. Although it is possible that Katherine could have joined her husband in Aquitaine, Weir considers this unlikely as wives rarely accompanied their husbands to war. In addition, someone had to manage the estates in England and take care of small children during Gaunt's absence. In a petition to the Pope dated 1 September 1396, John of Gaunt indicates that the affair began when he was married to Constance of Castile, and Katherine was free from marriage. Therefore the affair, according to him, began after the death of Katherine's husband. Based on this, Weir concludes that most likely the love affair began in the late autumn of 1372, when there was a significant increase in Katherine's social status in the Lancastrian household.