Dynamic scaling out and then back in is what defines cloud computing. However, the algorithms currently available still require some planning if you know certain days, weeks, or months of a year will require uncharacteristically higher capacity of resources. Given a sudden deluge of new traffic, your infrastructure will attempt to dynamically scale out, but if the rate of increase in traffic is logarithmic, even an optimized server utilization target won't help. Servers will frequently reach and operate at 100% utilization, resulting in dropped or erroneous requests. To prevent this from happening, you should proactively provision additional capacity during such predictable periods of high demand.