I am going to land at Krungthep on November, 19 in the morning. Then I plan head to ChiangMai where rent a car and take ride ChiangMai - Pai - MaeHongSon.
I would like to ask your advise: where is the best place for celebrate Loy Krathong? May be I should make stopover in Ayuthaya? Or go directly to ChiangMai and celebrate Loy Krathong there?
Or any other variant?
Thanks in advance for you suggestions.
Chiang Mai would be fine, particularly along the river, but it will likely be crowded and touristy too. If you are heading that way anyway, I might try Khamphaeng Phet. It's about the midway point between BKK and Chiang Mai, with a good section of river flowing past town, extra friendly folks, and hardly any western tourists. I don't think you'd go wrong there.
Phitsanulok and Lampang might also be good spots. They are Thai (as opposed to western) tourist towns, with great infrastructure and restaurants and decent enough sections of river for floats.
The November full moon is Sunday the 21st so Loy Krathong festivities will be mainly that weekend. If you're arriving in BKK the 19th and heading straight north I would do the festival first and leave the drive to Pai/MHS/etc for the following week. Chiang Mai is the top tourist spot for Loy Krathong, with Sukhothai a close 2nd. The other towns mentioned above are good alternatives, and Tak is a great Loy Krathong setting too right on the Ping river.