Cia's birthday on her earth is around 15th day of 6th month which is winter over there (note that Cia started her school 6 months ago which is around last year last month, 10th month was Goin on, so right now in story (chapter 23 or so) the months is 5th ending.
According to our earth , her birthday is around July 8th or so since the length of year is same but days are complicated and have 10 months in a year, meaning earth rotates around sun at same speed but it's own rotation is somewhat slow there.
So wish her on July 8th if you ever want!