WhatsApp doesn't read your conversations for ads because it can't. Message content is end-to-end encrypted. You must have leaked your plans through some other medium inadvertently.
This is called retargeting and is even worse from a privacy perspective: now your data is going through multiple layers of ad buying tools, analytics, ad exchanges, aggregators, ad networks, etc.: http://2bd2y2367xnj3kpntjsifzmf-wpengine.netdna-ssl.com/wp-c...
https://en.wikipedia.org/wiki/Behavioral_retargeting