At which stage legality check is conducted?

Republic Act No. 6735 'The Initiative andReferendum Act'

http://www.chanrobles.com/republicactno6735.htm#.UMhZ8Gd32uI

Sec. 8. Conduct andDate of Initiative or Referendum. — The Commission shall call and supervise theconduct of initiative or referendum.

Within a period ofthirty (30) days from receipt of the petition, the Commission shall, upondetermining the sufficiency of the petition, publish the same in Filipino andEnglish at least twice in newspapers of general and local circulation and setthe date of the initiative or referendum which shall not be earlier thanforty-five (45) days but not later than ninety (90) days from the determinationby the Commission of the sufficiency of the petition.