Is there a January Effect? What might explain the difference in the month effect between the first part of the sample and the second part of the sample?

Is there a weekday effect in either index? What might explain the weekday effect?

Monthly returns 1927-1997
S&P 500		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		-9.660E-05	1.446E-03	0.011398764	-0.000856461	0.010099	0.018598809	0.011065577	-0.011244705	0.000627294	0.005106719	0.013911263	0.016532927
Monthly returns 1998-current
S&P 500		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		-0.004556077	0.009606022	0.025327385	0.003687689	-0.00788	0.016380364	0.004384019	-0.012141311	0.022306256	0.020502141	0.00857903	0.002974328
Russell 2000		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		0.002335916	0.006520634	0.017859879	0.0050963	0.008306	-0.001325829	0.000939734	-0.00625633	0.009006699	0.02650154	0.024063545	-0.002641638
Weekday returns
S&P 500		M	T	W	R	F
		0.000872633	0.000505366	0.000195595	0.000226056	0.000419
Russell 2000		M	T	W	R	F
		0.000783686	0.000409114	0.000443616	0.000576277	-0.0003
Monthly returns, enitre History
S&P 500		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		-0.000160613	0.002992925	0.01294267	-0.000646186	0.007222	0.015811641	0.00718135	-0.016183883	0.005176719	0.00853357	0.014144161	0.011764618
Entire Monthly returns		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		-0.02%	0.30%	1.29%	-0.06%	0.72%	1.58%	0.65%	-1.65%	0.52%	0.85%	1.41%	1.18%
Returns 1927-1997		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
S&P		0.02%	-0.02%	1.11%	-0.04%	0.95%	1.81%	0.89%	-1.10%	0.03%	0.34%	1.41%	1.63%
Returns 1998-current		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
S&P		-0.16%	1.11%	2.49%	0.27%	-0.60%	1.53%	-0.17%	-1.21%	2.23%	2.05%	0.86%	0.35%
Russell 2000		0.52%	0.79%	1.73%	0.27%	0.80%	-0.46%	-0.72%	-0.63%	0.90%	2.65%	2.41%	-0.32%
Weekday returns
S&P 500		M	T	W	R	F
		0.09%	0.05%	0.02%	0.02%	0.04%
Russell 2000		M	T	W	R	F
		0.08%	0.04%	0.04%	0.06%	-0.03%