The phrase "gala days" typically refers to days or events of great celebration and festivity. There are a variety of synonyms that can be used to convey the same meaning, however. Some possibilities might include "festive occasions," "joyful commemorations," or "mirthful gatherings." Other options could include "grand festivities," "merry-making events," or "celebratory occasions." Depending on the context, some additional synonyms might include "festive milestones," "revelrous holidays," or "exultant festivities." Ultimately, the choice of synonym will depend upon the specific connotations and tone that the writer wishes to convey.