"Every 30 days"?

It seems that you don't live near a coast, or don't pay attention to tides. 

"Spring" tides occur twice a month, at new and full moon. In the Northern Hemisphere the new moon tides go higher, and lower, during the summer and in winter the full moon tides are the most noticeable. That relates to the northern hemisphere being closer to the new moon during summer days and closer to the full moon on winter nights.

https://oceanservice.noaa.gov/facts/springtide.html