What exactly is the difference betweekn 記念日 and 周年? I found both to mean "anniversary". Thank you in advance.
周年 just indicates the number of years that something has been in continuing or in operation. This term is used in regards to anniversaries, but only as a description of what the anniversary is for.
記念 is remembrance or commemoration. Adding 日 to it makes it 'Commemoration Day', or 'Anniversary'.
These two terms are often used together to indicate both the celebration/commemoration and what specifically it is celebrating.
10周年(の)結婚記念日. Day of celebration of 10 years of marriage; 10-year (marriage) anniversary.