This is a Phase 1a/1b, open-label, dose escalation and expansion study to evaluate the safety and anti-tumor activity of DAY301, a PTK7-directed antibody-drug conjugate in participants with advanced or metastatic solid tumors. The study comprises of 2 phases: Phase 1a dose escalation where participants will be administered DAY301 at escalating dose levels to assess safety and tolerability, and to determine the maximum tolerated dose (MTD) and/or the recommended dose (RD); In Phase 1b dose expansion, DAY301 will be evaluated in dose expansion cohorts.