Final answer:
On the first morning after Gawain's arrival, the lady of the castle invited him to see the garden.
Step-by-step explanation:
The lady of the castle invited Gawain to come and have a look at the garden on the first morning after his arrival. Gawain thought it was nighttime but decided to go anyway since no one else was awake. This incident occurs in the poem 'Sir Gawain and the Green Knight,' written by an anonymous author in the 14th century.