On the MAIN page there's a link to 2017 which can take you to 2016 and so on. All the rides are posted for each year. Of all the parties we've attended (last week in October) 2017 was the first year we've ever ridden BTMR.
Refurbishments a look see...
Splash Mountain
August 28th through November 16th,
2017
January 8th through February 1st,
2018
Big Thunder Mountain Railroad
April 4th through to July 15th,
2016
August 8th through November 18th,
2016
Based on this information, it looks like
Big Thunder Mountain Railroad is in need of refurbishment.
EDIT:
MNSSHP 2016 - Open Attraction in Frontierland
Frontierland
Country Bear Jamboree
Splash Mountain
Liberty Square
Haunted Mansion
MNSSHP 2017 - Open Attraction in Frontierland
Country Bear Jamboree
Big Thunder Mountain Railroad
MNSSHP 2018 - Open Attraction in Frontierland
Frontierland
Country Bear Jamboree
Splash Mountain
Do you all see the pattern here?
Some party goers were able to ride both just prior to one of them shutting down for refurbishment.