WITH range(1,12) as months

FOREACH(month IN months | 

    CREATE (m:Month {month: month})

    FOREACH(day IN (CASE 

                      WHEN month IN [1,3,5,7,8,10,12] THEN range(1,31) // these months have 31 days

                      WHEN month = 2 THEN range(1,28) // not working with years, so no leapyear calc, default Feb has 28 days

                      ELSE range(1,30) // all other months have 30 days

                    END) |      

      CREATE (d:Day {day: day})

      MERGE (m)-[:HAS_DAY]->(d)))