The word "opportunistic" have always had negative connotations. It implies that you're taking advantage of something or someone in a negative situation.
But it's not bad to be opportunistic when marketing your brand. As long as your message is suitable* for the occassion, then it's the OH YOU SMART ALEC thing to do!
Just look at Nandos.
It's smart, it's humorous and it's RELATABLE.
This one simple advertisement allowed Nandos to build rapport with their target market - the young 'uns who are feeling the pinch of today's economy.
And they're consistent with their messages. You can always count on Nandos to have an opinion about current issues, and you can count on them to be cheeky about it. It has become part of their branding.
Now Nandos is the cool brand and relevant brand. Relevant because they so obviously took the effort to talk about the issues that their customers are struggling with. They show that they care. And they get publicity for it. That's good for business.
So really take time to consider what interests your target customers share, and take opportunities that relate to that. It can be as simple as knowing when 'World Friendship Day' is and crafting a message that acknowledges this + reflects your brand.
*suitable = not unreasonably controversial or inflammatory