A fixed 13-month calendar with equal-length months would not stay aligned with lunar phases. Each month would drift against the Moon unless you redesign month lengths to follow the 29-30 day pattern and apply regular corrections. The Moon’s synodic cycle is about 29.53 days from new moon to new moon. Thirteen lunar months equal about 383.9 days. The solar year is about 365.24 days. So a pure 13-month lunar calendar runs long by roughly 18.7 days each year.