Not everything's open every day in Rome. The Vatican museums and Sistine Chapel close one day a week; so do lots of favorite restaurants and shops.
So when planning your trip to Rome, it pays to have a basic idea of what day in the week is best for which sight or activity. Here's help. (In the form of a "rhyme." Move aside, Shakespeare Keats Dr. Seuss Eddie Mannix).And here it is... in video form!
Dreaming of the Sistine Chapel? Then don't go on a Sunday
The Vatican museums (which include the Sistine Chapel) are open every day but Sunday. On the last Sunday of the month, they are open and free, but it's not something I recommend if you value your vacation time; the line is often three hours or more (and you can't book a ticket in advance on the Vatican website). St. Peter's Basilica is open daily; on Sunday, the Pope appears at 12pm to an audience on the square, and on Wednesday, he has his general audience at 10:30am.
Best time to go to the Vatican: Wednesday morning, as the museums tend to be emptier while the Pope does his audience; otherwise, Tuesdays, Thursdays or Fridays, since Saturday and Monday tend to be crowded with people who would have gone on Sunday.
And stay away from smaller churches—at least if it is midday
Most churches are open daily in Rome. However, many of the more off-the-beaten-path churches also close midday, some for as long as from 12pm to 4pm, so always check. On Sunday, remember that they may be holding Mass and more ceremonies than usual, which can make it more difficult (or forbidden) to walk around to sightsee.
Best time to go: Morning or evening, except for Sundays (unless you want to see Mass).
On Monday, many museums are a no-go
Most of Rome's best museums close on Mondays, but are open every other day of the week. These include the Borghese Gallery, Palazzo Barberini, Palazzo Massimo, Crypta Balbi, MAXXI, Castel Sant'Angelo, and the National Gallery of Modern Art.