To correct the error in the sentence "Today is Mondy," you would use the proofreading mark for a spelling error, which is generally represented by a caret (^) to indicate where to insert the missing letter. In this case, you would place the caret just before the "n" in "Mondy" and write the letter "a" above it to indicate that "Monday" is the correct spelling.
So, the marked sentence would look like this:
Today is Mon^a dy.
Alternatively, if you prefer to indicate the correction more simply, you could cross out "Mondy" and write "Monday" above it.