Every year for the last 4 years - I have checked my <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period apps</a>

December and January every year for the last 4 years I have been late on my period ranging from 10-20 days. Is this normal? It’s only ever December and January. Whereas any other month it can go from 2-3 days early or 2-3 days late...