As everyone else has stated "Summer". (Tires have higher pressure, shorter cold-start fuel enrichment, thinner trans fluid/axle grease/PS fluid/oil/air...)
And the change from (probably) bulk 5w30 kwik-lube oil to PU 5w20.
Go ahead and change the plugs, but in my experience stick with OE brand/style of plugs. Only deviate from stock brand/style of plug by getting better plugs within the same family i.e.: If your OE plug spec is Bosch double platinum with a conventional single ground-strap then use a Bosch double platinum or Iridium plug with a conventional single ground-strap. Do not use quad ground-strap style or splitfire, or autolite or Bosch copper plugs unless you genuinely have a special scenario and you really know what you're doing! (Special scenarios would include highly modified engines including turbos, nitrous, or high-compression pistons... then maybe the best plug is 100 miles away from stock like an Autolite copper plug or Denso "U-Groove" plug, or Bosch quad ground-strap plug.)
Even better than new plugs, new upstream (pre-cat) O2 sensors (again OE brand) will probably increase your gas mileage measurably. O2 sensors "work" well beyond 100K miles, but "working" and "optimum performance" are two different animals. Oil still "works" beyond 20K miles, but you probably don't let it go that far before changing it because you don't want "working" you want somewhere between "optimum performance" and "working well". A 13" black-and-white television "works" with an antenna, but a 60" 4K OLED TV fed by cable or satellite is nearer to "optimal performance".