Pondělí is singular, so why is it plural in the answer?
Pondělí is plural as well: http://prirucka.ujc.cas.cz/?slovo=pond%C4%9Bl%C3%AD
Why isn't the expression "I hate Mondays" allowed?
Also, the construction used -- mít rád -- is generally translated as "like," while "hate" has its own Czech word, "nenávidět".
I think it is too strong.
I think it's a bit too strong. (Also, I don't like Mondays either.)
Mondays always take a capital "M"?
Yes, in English they do.
Same as Thursday, or March, or September. They're all proper names in English.
In Czech, OTOH, they are "pondělí", "čtvrtek", "březen", "září", and "angličtina" -- normal nouns with lower case letters.