RU-ALLRUSSIAN 1982 Problem 5

A library is open every day except Wednesday. One day three boys, A, B, C visit the library together for the first time. Thereafter they visit the library many times. A always makes his next visit two days after the previous visit, unless the library is closed on that day, in which case he goes the following day. B always makes his next visit three days after the previous visit (or four if the library is closed). C always makes his next visit four days after the previous visit (or five if the library is closed). For example, if A went first on Monday, his next visit would be Thursday, then Saturday. If B went first on Monday, his next visit would be on Thursday. All three boys are subsequently in the library on a Monday. What day of the week was their first visit?