"It is possible to visit it on the first Sunday of the month, from May to October."
Here we go:
C'est possible : this is not grammatically wrong, but it is usual to use the impersonal "il" = "il est possible"
DE le visiter (like: possible TO visit)
"on the first Sunday" does not directly translate word to word. When a date is given (on May 1st, on the first Sunday of the month), the French just use the definite article "le" or "la": --> le premier dimanche du mois (on the first Sunday of the month) --> le premier mai (on May 1st) --> la dernière nuit de juin (on the last night of June)