Transition in v-for with VueJS

You have to use TRANSITION GROUP, not TRANSITION.

I would suggest to use something like: https://github.com/asika32764/vue2-animate which is great package which allows you to use CSS animations from AnimateCSS in your Vue application.

For example, in your case you would use something like that:

<transition-group name="fadeLeft" tag="div" >
    <img v-for="day in days" :src='day.preview'  v-bind:key="day" v-if='day.current' title='Preview'>  
</transition-group>

You're doing v-if="day.current" so I doubt that's ever going false long enough for Vue to recognize it to do the transition. If you just add :key="day.preview" to your image tag you'd be good to go.

https://jsfiddle.net/hfp78gfe/1/