Is there really a best day of the week to book airfare? What about time of day? If you have a little flexibility in your travel plans, you can usually snag a pretty good ideal.

Here are some commonly asked travel questions that can be a little tricky, even for the most seasoned of travelers.

1. I bought my tickets, then saw a lower fare. Can I change my ticket?

You can, but it may cost you a pretty penny because most airlines charge a change fee. Here are the exceptions:

--Change within 24 hours: The Department of Transportation requires U.S. airlines to allow a 24 hour grace period after booking in which travelers can change (or cancel) flights at no charge.

--Southwest: This is the only U.S. airline that doesn’t charge a change fee; if you see a lower fare, you can simply rebook.

--Refundable ticket: If you bought one of these fares, you can change without penalty. But these refundable fares tend to be very expensive.

FRONTIER, SILVER LATEST AIRLINES TO SUSPEND REGULAR FLIGHTS TO CUBA

2. What are the cheapest days to fly?

In the U.S., these are usually Tuesdays, Wednesdays and Saturdays; for international flights, it's usually cheaper to fly weekdays instead of weekends. Notice the word “usually"-- there are occasional exceptions to this rule, which is why you must always compare airfares by prices and travel dates.

3. Carry-ons are always free...right?

Not on all airlines. Ultra-discount carriers around the world often charge for a standard-size carry-on, though some like Spirit allow for a very small bag if placed under a seat. In general, older, large airlines (sometimes referred to as legacy carriers) allow free carry-ons. As for big checked-bags, all airlines charge a fee with one exception: Southwest

4. Why are security lines always so long?

They aren’t always super long, but you can check to see wait times with the TSA app or online. If you really want to move quickly through security, join TSA PreCheck with its dedicated travel lines, or if you fly internationally, choose Global Entry which includes PreCheck. You’ll pay for both, but it’s just $20 or less over the course of five years which is how long memberships are good for.

5. I haven’t flown in a while, what happened to my favorite airline?

If you’re referring to AirTran, Continental, Northwest or US Airways, they’ve all disappeared into mergers. Virgin America is next, now in the process of being merged with Alaska.

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6. Which airline has the cheapest fares?

No airline always has the cheapest fares because of today’s extremely competitive marketplace, but finding a great deal online is simple: always compare fares (on this comparison site or any other). There is no other way to be sure of always getting the best price.

7. Why are airfares so expensive?

They’re not so high, not all the time. Try to avoid flying when most people want to fly because popular times (such as mid-morning) and popular days-- Fridays, Sundays-- tend to be more expensive. Instead, fly the cheapest days (see #2) and cheaper times (often at the crack of dawn, noon time, overnight flights). If you can fly at least one leg of your trip on a less popular day/time, you’ll still save something.

Now, let’s talk about Europe; these are roundtrip fares for Boston-London flights through the years.

--Nov. 2013, $888

--Nov. 2016, $555

--May 2017, $415

Nice, huh? Plus I just saw Boston-Brussels for under $370 (found on my site, March 13). As for domestic fares, an airline lobbying group reached way back to compare average fares for 1941 to 2015 for Boston-Los Angeles flights and (with prices adjusted for inflation) found that fares dropped over the years from $4,540 to $481.

Want to travel that same route in May of this year? You can go nonstop for as little as $313.