Big Thunder Mountain Railroad will be closed for refurbishment from the 4th – 21st May, 2020 at the Magic Kingdom, Walt Disney World Resort, according to internal calendars.
It doesn’t seem to be a long refurbishment, so it may just be a maintenance refurbishment. However, stay tuned to Orlando Theme Park Zone for the latest.
EXTRA: It should be mentioned that these scheduled downtimes for attractions are subject to change without notice and could be extended, shortened etc.
Source: Blog Mickey