As in rainy season most part of the day is cloudy without sunlight, it is not recommended to do nasya in such case.
However you can do Anu Taila Nasya which is recommended for daily use in Ayurveda.
Next Steps
If it rains on that day and its cloudy then avoid Nasya.
you can do nasya when there is sunlight.
Health Tips
After Nasya avoid headbath, walking in windy & dusty places.
Perform gargling with hot water after Nasya.
Have food only after an hour of the procedure.