Grammatically, it is wrong to assume that a preposition should always be followed by nouns (or noun phrases), clauses or gerunds. It could be followed by a prepositional phrase. For example:
From behind the cloud appeared the sun.
In the above sentence, both "from" and "behind" are prepositions and "behind the cloud" is a prepositional phrase functioning as a complement of the first preposition "from".
In your example, "on Sunday" is a prepositional phrase which is acting as a complement of the first preposition "about". It doesn't have any grammatical issue.
Also, you could consider a gerund phrase (going to the movies) is omitted because it was mentioned before. For example:
A: How about going to the movies on Saturday?
B. How about (going to the movies) on Sunday?
You can omit "on" in the second sentence:
How about (going to the movies on) Sunday?