“Keep in mind that it's a lot warmer there at this time of year.”

Your roommate is going on a trip for a few weeks to Bali, which she's never been to, but you have. You're giving her advice on what to pack for the trip. You don't think she needs to take a lot of heavy clothes because it's warmer there during this season than in the town where you live. You say:

Keep in mind that it's a lot warmer there at this time of year than it is here.

keep in mind that (clause)

"Keep in mind" means to remember. But the word "remember" can be used in a lot of different kinds of situations. "Keep in mind" is more specific. It means to remember a fact and continue to think about it while you're making decisions.

In the example above, the speaker thinks the listener already knows that it's warmer in Bali. She's saying "keep in mind that it's a lot warmer there" to remind the listener and tell her that she should use that information when she decides what to pack for the trip.

(this / that) time of year

Use this phrase when you're talking about things that usually happen during a certain season. It can be used when talking about weather, business, holidays, and other topics. For example:

The stores are always packed at that time of year.

It tends to rain a lot at this time of year.

Notice that the phrase is "that time of year", not "a year" or "the year".

it's (comparative adjective) (somewhere) than it is (somewhere)

Use this phrase to compare descriptions of two places. Some more examples:

It's less crowded there than it is in New York.

It's noisier in Calcutta than it is in Singapore.