For season 1, everything until Waterpark is during the school year. Then you've got a gap of at least 2 months between "Smunday" and "The bots and the bees". Assuming Francis came home with Lois on her flight and got the boys grounded later that day, at least two months have to have passed, and Smunday still has to be in June, implying that every episode up til "The bots in the bees" has to be before mid-april.
"Red Dress" took place right after Christmas since that's when Hal and Lois' anniversary is, but Pilot could have been much earlier in the year.
I should notice that "Vegas" marks the anniversary that Francis started working at the ranch...so it has to take place around the same time of the year as Zoo, which was late summer.
I think Grandma Sues was in late fall; Forbidden Girlfriend started "6 weeks into the year" so it couldn't have been in December.
The problem with Dewey's birthday is in season 6...seasons 1 and 4 implied it was in May or June, but season 6 implies it's between Pearl Harbor Day and Christmas...both of which are in December. Really, though, Kitty's Back doesn't fit well into season 6 at all...I mean, who sunbathes in December?
Francis' birthday is screwed up too over the years; it's shortly after thanksgiving in season 2 but well after Christmas and well before Halloween in season 6.
Also, Hal and Lois had 2 anniversaries in 2005

I assume Blackout takes place several months after "Halloween"