Impact of religious holidays whose dates shift in the Gregorian calendar I need to model and predict a quantitative variable that is heavily impacted by events, parties and celebrations. For I seek first to know the impact that has the religious festivals, the ramadan and the tabaski on my variable but the problem the date of these festivals are variable in the Gregorian calendar, therefore I do not know how to capture this effect.

If you are doing time series forecasting in the presence of such time-shifting predictors, then De Livera et al. (2011) would probably be of interest. Their proposed methodology (BATS/TBATS) aims at modeling and forecasting complex seasonality, and one of their applications actually is to Turkish electricity demand, which is heavily influenced by Eid al-Fitr and Eid al-Adha. An ungated version of the paper is available at Rob Hyndman's site, and their method is implemented in the forecast package for R.
