Comics related to Valentine's Day (celebrated on February 14) and giving Valentines. The only positive comic (up until the year 2015) was the 543: Sierpinski Valentine and all the other except 62: Valentine - Karnaugh, is directly negative about this day. Seems Randall doesn't approve of it much.
All the comics came out on Valentine's Day or on the closest day before that except the first year (2006) with two Valentine comics on both the 10th and the 13th of February
The first to come out on Valentine's Day was in 2007.
Then there where no comic in 2008. However, he did release a very depressing comic in 2008 on the 13th: 383: Helping. And similarly he released a very gross comic in 2011 on the 14th: 860: Never Do This. This could very likely be on purpose giving his possible negative feelings for this day.
Also because he did Valentines comics both the two years in between (2009 on the 13th and 2010 on the 12th), and the year after in 2012 on the 13th.
But since 2012 he has not released a Valentines comic, and there seem to be no relation at all with the comics that are coming out closest to this day, or the ones on either side of this day. The relevant comics are from 2013 February 13th: 1173: Steroids, 2014 February 14th: 1330: Kola Borehole, 2015 February 13th 1486: Vacuum, and 2016 February 11th 1642: Gravitational Waves.