i have a google sheet with two tabs. First tab is all the reservations data i have for a house i rent (First Name, Last Name, Check in Date, Check out Date etc...) and in the second tab is a calendar. I am looking for a way, when i add a new row in the reservations tab automatically that reservation to be added in the calendar in the correct days.

This the google sheets link

For calendar, put this function in B2 (no merged cells anymore except the title), nothing in C2, D2 ... and format as dd

=sequence(1;datedif((A1&"-"&vlookup(B1;'μήνας'!A:B;2;0)&"-"&1);eomonth((A1&"-"&vlookup(B1;'μήνας'!A:B;2;0)&"-"&1);0);"d") 1;(A1&"-"&vlookup(B1;'μήνας'!A:B;2;0)&"-"&1))

Finally, your function is in B3

=if(SUMPRODUCT((RESERVATIONS!$A$2:$A=$A3)*(RESERVATIONS!$D$2:$D<=B$2)*(RESERVATIONS!$F$2:$F>=B$2));"x";)

drag to the right and below