We propose a power-law decay model with autocorrelation for posting data to social networking services concerning particular events such as national holidays or major sport events. In these kinds of events we observe people's interest both before and after the events. In our model the number of postings has a Poisson distribution whose expected value decays as a power law. Our model also incorporates autocorrelations by autoregressive specification of the expected value. We show that our proposed model well fits the data from social networking services.